Why Doom Execute Is the Only S+ Answer to Co-op HP Scaling
Your four-player lobby walks into a Test Subject #C10 and the health bar stops looking like a number from a video game. The bestiary lists 100, 200 and 300 HP across three phases, and co-op scaling multiplies that base 600 by four players and by the 1.3 Act 3 boss coefficient, which lands somewhere around 3,100 effective health. The Ironclad beside you now needs roughly five times the raw damage he needed in solo. Your Doom counter needs the same number of casts it always did. That gap is the entire reason this build exists, and it is why our build page rates Doom Execute as the only S+ archetype in multiplayer.
The scaling rules are worth memorizing because they shape every co-op decision you make. Per the co-op guide, enemy HP equals base HP times player count times an act coefficient: 1.1 in Act 1, 1.2 in Acts 2 and 3, and 1.3 for the Act 3 boss. Four players therefore face roughly 4.4 to 5.2 times the health bar. Enemy attack damage does not scale with player count, but Plating, Artifact, Slippery and Skittish stack higher, and enemy Block scales too. Multiplayer inflates the target and its armor while leaving the incoming damage flat, which is exactly the world where a percent-based execute is prized and raw damage decks drown.
In solo the deck still earns its S rating, with a difficulty rating of 3 out of 5 on the builds page. The honest framing is this: it is slower than a Claw deck at deleting 42 HP trash in Act 1, it asks you to manage Souls as a second resource, and its payoff is the most reliable late-game scaling of the five characters. Scourge and Countdown keep producing the same Doom per turn against a 234 HP Soul Nexus, a 300 HP Mecha Knight or the Queen at roughly 400 HP behind 199 guard, fights where flat damage decks start doing arithmetic on their remaining HP.
The core listed on our builds page for this archetype is Blight Strike plus End of Days plus a Soul engine, and the page attaches one warning worth repeating here: balance Soul generation against Soul spending, or you stall out in boss fights. Everything in this guide serves that balance. Numbers reflect v0.111 beta on the beta branch; the v0.108 patch reordered end-of-turn resolution so Doom no longer kills first, and that single line changed how the execute check is played around, which is where the mechanics section starts.
How the Execute Check Actually Works, and the v0.108 Order Change
The glossary definition is short: Doom is a stacking debuff, and at the end of the turn, any enemy whose Doom count is greater than or equal to its max HP is executed. Read that twice, because two words do all the work. It is a threshold check on a debuff, not an attack, and it reads max HP, not current HP. Everything exploitable about this build flows from those two properties.
First, the not-an-attack part. Mechanics that tax attacks do not tax the check. Skulking Colony caps the damage it takes per hit, which the bestiary says hard-counters single big hit decks, and its counter note states that damage-over-time effects bypass the cap: Doom sits squarely in that bypass category. Waterfall Giant is the showcase. At 0 HP it turns invulnerable, clears its debuffs, and self-destructs the following turn for damage equal to its Steam Eruption stacks, unless it dies at zero stacks, and the bestiary explicitly names a Doom execute as the example kill. One detail from the same entry: Weak does not reduce the blast. If the fight somehow reaches the explosion turn, block for everything or accept the hit, because your mitigation debuff is useless there.
Second, the timing part, which is where v0.108 matters. The patch notes for that version say the end-of-turn resolution order changed and Doom no longer kills first. The practical consequence is simple: on the turn you push an enemy to threshold, plan for that enemy to still resolve its own end of turn. Do not spend your last energy pushing Doom to exactly the execute line and assume the fight ends before the incoming swing lands. Block that turn, or have Defy's Weak up, and treat the execute as arriving one exchange later than your instinct from older patches says.
Last, target discipline. Doom sources split into targeted application, like Scourge and End of Days, and random application, like Countdown, which applies its Doom to a random enemy at the start of each turn. In single-target boss fights the difference is invisible. Against Entomancer summoning bees all fight, or the four bodies of the Phantasmal Gardener, random Doom is diluted damage. The rule: when you cannot control the target, control the fight length, and burn the smallest bodies first so the random ticks concentrate on what remains.
Scourge, Countdown and End of Days, Card by Card
Scourge is the archetype's workhorse: a 1-cost common Skill that applies 13 Doom and draws 1 card, or 16 Doom and draws 2 once upgraded. Its card text says it never costs tempo, and that is literal: it replaces itself or better, so casting three in a turn costs you three energy and zero cards of hand. In a deck where the win condition is a stack counter, a card that grows the stack while refueling is the definition of the engine. Two copies cycled per shuffle means 26 Doom per pass unupgraded, 32 upgraded, before any other source contributes. It is the first card you should take in almost every Act 1 reward, and its upgrade is a top-three campfire priority all run.
Countdown is the passive layer: a 1-cost uncommon Power that applies 6 Doom at the start of each turn to a random enemy, 9 when upgraded. Run the numbers on a six-turn boss fight and one unupgraded copy contributes 36 Doom for a single card and a single energy paid once, upgraded 54. Two copies compound. The more valuable property is what the card text calls a passive Doom engine: it works during turns you spend entirely on blocking, during Knowledge Demon's card and energy caps, and through Ceremonial Beast's Ringing phase where you may only play 1 card per turn. Decks that win through plays get choked by those fights. A percentage of your output does not.
End of Days is the 3-cost rare finisher: massive AoE Doom that immediately executes enemies at threshold. The word immediately is the differentiator from every other Doom source, because the normal check waits for end of turn. End of Days resolves on cast, which lets you clear a board in the middle of an enemy's lethal setup. Its natural habitat is wide fights: Phrog Parasite triggers Infested on death and spawns four Wrigglers, Entomancer summons bees continuously, Ovicopter sits behind eggs that hatch within a turn or two. One End of Days prices every body on the field. Upgrade it before any elite path with a wide pool, and against bosses treat it as your threshold closer once Scourge and Countdown have done the stacking.
Reaper Form gets a full treatment in the pivots section, but note its line now: a 3-cost rare Power where whenever your attacks deal damage, you apply that much Doom. It is the ceiling card of Doom and attack hybrids, which means Osty, covered next, is secretly part of this card list.
The Soul Economy: Reave, Haunt and Why Boss Fights Stall
Souls are the Necrobinder's second currency, and the character page defines them precisely: 0-cost tokens that draw 2 cards and then exhaust. Reave is the baseline generator, a 1-cost common Attack that deals 9 damage and shuffles 1 Soul into your draw pile, upgraded to 11 damage and 2 Souls. Played correctly, Souls do two jobs at once. They cycle your deck mid-combat, and because they exhaust on resolution they remove themselves, thinning the deck for the rest of the fight and raising the density of Scourges you draw on the next pass. A Soul is the rare resource in this game that pays you and then cleans up after itself.
Haunt converts that cycle into damage: a 1-cost uncommon Power where whenever you play a Soul, a random enemy loses 6 HP, 8 upgraded. Our card database calls it the pivot of the S-tier Soul engine, and the reason is compounding: each Soul played is a free card that draws two, so a Reave-fueled hand can resolve several Souls in one turn, each one triggering Haunt while feeding you the next card. The engine plays fast, deals its damage as direct HP loss, and asks almost nothing in energy.
Now the warning from the builds page: balance Soul generation against Soul spending, or you stall in boss fights. Understand the failure mode concretely. Souls draw cards but apply no Doom. Against a 321 HP Knowledge Demon you need Doom density, and a hand cycling Soul into Soul produces velocity without progress: you take your turns, your hand empties, the Doom counter barely moves, and the boss debuffs keep stacking every round. Velocity feels like winning and executes nothing.
The working rules from play: take Reave freely in Act 1 because it is a fine damage card while the deck is still random, and because Souls smooth out awkward hands against Fossil Stalker and Haunted Ship. From Act 2 onward, stop taking additional Reaves unless Haunt is already in the deck. In boss fights, spend Souls to dig for Scourge and Countdown, not for their own sake. Two co-op notes complete the picture: the v0.108 co-op card Soulbound links a teammate so your Souls enter their deck too, which turns the engine into shared fuel, and since v0.109 nearly any potion can target teammates, so your draw potions can bail out the player holding the front line.
Osty the Skeletal Hand: Body-Blocking, Unleash and the Summon Package
Osty is not a pet, it is your second health bar and your second win condition. The glossary defines it as the Necrobinder's summoned skeletal hand that attacks on command and body-blocks for the team, and Summon is the keyword that adds or restores its HP. Your starter relic, Bound Phylactery, Summon 1 at the start of each turn, means Osty is growing passively in every fight of the run without you spending a card on it. On the roster's lowest starting HP at 66, that recurring body between you and the enemy intent line is the difference between taking elite paths and dodging them.
The package has three moving parts. Bodyguard is the 1-cost basic Skill that Summons 5, upgraded 7: the baseline growth spell, and early on it is a legitimately strong play because Osty's HP pool is both defense and ammunition. Unleash is the payoff, a 1-cost basic Attack where Osty deals 6 damage plus damage equal to its current HP. Read that scaling again: every point of Osty HP you banked becomes damage on cast, so the card text is right that the bigger Osty gets, the harder it hits. A Bodyguard or two in the mid game turns Unleash into one of the most efficient damage cards available to this character. Poke fills the gaps: a 0-cost common Attack where Osty deals 6 damage, 9 upgraded, free real estate on any turn where you paid energy for Scourge and want the damage to keep moving.
The body-blocking half deserves its own planning. In solo, Osty standing in front of you means a 12-damage Nibbit ram or a Fossil Stalker bite has a second possible answer besides your Block cards, which matters enormously against Suck, where every unblocked hit grants the Stalker 3 Strength. In co-op, the role guide casts the Necrobinder as board control whose Osty soaks hits for the team: with four players sharing one enemy action sequence, a body that eats swings for the Ironclad holding the line is worth more than any individual damage card.
The discipline point: every Osty card is a card not applying Doom, and this is a Doom guide. Treat the package as the bridge and the shield, not the destination. The correct ceiling version of Osty is the hybrid one, where Unleash swings connect through Reaper Form and each point of damage becomes another stack of Doom. That version gets the best of both resources, and it is the strongest shape this archetype takes in Act 3.
Assembly Order: Draft Picks from Floor 1 to the Act 3 Boss
Act 1 rewards set the tone. Priority one is Scourge, and it is not close: 13 Doom plus a draw at 1 cost is the best common in the character's pool for this deck, and stacking copies early is how Byrdonis (91 HP) and the Phrog Parasite elite windows stay winnable. Priority two is Defy, the 1-cost common Skill applying Weak for minus 25 percent enemy damage, which the database rates as the best-value mitigation in the game. One copy rewrites elite math: Byrdonis alternating Dive and Peck, the Mawler's 14-damage Tear after its roar applies 3 Vulnerable, and Bygone Effigy's wake-up turn all shrink against a Weak. Priority three is a single Bodyguard or Reave as bridge value while you wait for the engine, and Poke is a fine floor 1-to-5 pick because free damage ages well all run.
Map your Act 1 elite choices to what Doom actually does. Bygone Effigy cycles between asleep and awakened, and the sleep phase is your free setup window: apply Doom, grow Osty, set up powers without pressure, then hold Block for the wake-up turn. Byrdonis is a race where Weak on Peck turns plus steady Scourges outpace its loop. Phrog Parasite alternates Infect and Lash and only ends when all four post-death Wrigglers die, so if you already have End of Days, bank it for the Infested phase instead of spending it on the body.
Act 2 is where the archetype locks in. The shopping list, in order: Countdown copies, End of Days, and a judgment call on Haunt based on how many Reaves you are carrying. The Infested Prism at 200 HP is the act's litmus test, and the act notes are blunt: have your engine online before Act 2 or skip elite paths. If by floor 8 you are still winning with Reave and Poke swings, prioritize rest and upgrade over greed. Act 3 wants the ceiling: Reaper Form if offered, a second Countdown, and enough defense to survive Soul Nexus opening with Soul Burn for 29 on turn one.
Removals and upgrades follow one principle: this deck wants density, not width. Remove starter Strikes before Defends, because Osty and Scourge carry your damage while nothing else carries your 66 HP. The campfire order is Scourge, then Countdown, then End of Days, and remember the beginner guide's point that upgrading a key card often outvalues healing: HP above 1 is spare, and a 16-Doom Scourge that draws 2 compounds every single turn it is drawn for the rest of the run.
Act by Act: Every Boss Window Where Doom Wins or Dies
Act 1a bosses. Vantom opens with 9 stacks of Slippery, every hit deals only 1 damage, on a fixed 4-turn loop where turn 3 is a 25-plus killer swing and turn 4 self-buffs. Your main engine does not care about Slippery's attack tax because Doom application is not an attack; strip stacks with Osty commands if you want chip, full-block turn 3, and take the kill inside the turn 4 window. The Kin is target practice for focus discipline: acolytes hit hard and scale Strength, the priest debuffs from the back, so burn the low-HP acolytes first and let concentrated Doom finish the priest. Ceremonial Beast is the marquee matchup: it gains 2 Strength every turn in a pure DPS race, staggers below 150 HP, then applies Ringing where you may only play 1 card per turn. Countdown ticks straight through Ringing, which is why this boss is the single best argument for the passive engine.
Act 1b bosses. Waterfall Giant stacks Steam Eruption on almost every action and self-destructs at 0 HP unless killed at zero stacks: the Doom execute is the clean answer the bestiary itself suggests, but mind the v0.108 ordering and survive the exchange where you push it over. Soul Fysh stuffs Beckon statuses into your deck, 1-cost cards that deal 6 unblockable damage if left unplayed; your Souls cost 0 and keep hands playable, and its Intangible on turns 4 and 5 barely dents you for the same reason the execute ignores attack taxes. Lagavulin Matriarch starts asleep and worsens as the fight drags: set up Countdown and Osty during the sleep, then push.
Act 2 bosses. Knowledge Demon forces a permanent debuff choice every round: 6 damage per turn, minus 1 draw, later card and energy caps, while healing itself and gaining Strength. The draw cap is aimed directly at card-driven decks; Countdown is your answer that no debuff choice turns off. The Insatiable runs a Sandpit death timer starting at 4 where each Frantic Escape card buys a turn: passives keep executing the fight's real work while your plays service the timer, a near-per Doom fit. Kaiser Claw pairs two bodies, the Rocket claw's 31 Laser being the lethal move: kill both within 1 or 2 turns because the survivor gains Strength and 99 Block.
Act 3 bosses. C10 is the counterweight: DoTs reset between phases across its 600 total HP, and phase 1 punishes skills with 2 Strength, taxing Scourge and Defy, so lean on Countdown and Osty attacks, block perfectly in phase 2, and strike only on vulnerable turns in phase 3. Doormaker is friendlier: the bestiary says DoT decks shine across its phases and the phase-2 stun is the window. The Queen limits you to 1 Bound card per turn while mid-turn draw stays unrestricted, so dig mid-turn for the finisher and kill the Torch Head Amalgam before her Enrage loop starts. Aeonglass was nerfed in v0.109, so verify its current numbers in-game before the finale.
Relic Fit: Beating Remnant, Ectoplasm and the Stall Package
One relic is so on-theme that our own synergy page names it: Beating Remnant, a Rare that caps the HP you can lose per turn at 20. The entire game plan of this deck is staying alive long enough for a threshold check to fire, and a hard cap on per-turn damage is the stalling mathematical identity of that plan written in relic form. Against Owl Magistrate's Verdict for 33 plus 4 Vulnerable, or Soul Nexus's opening Soul Burn 29, Beating Remnant converts a lethal turn into a survivable one. If an Ancient or shop offers it, the decision takes two seconds.
The rest of the stall package ranks by how much effective HP it returns. Pantograph heals 25 at the start of boss combats, and this is a deck that defines itself in boss combats. Red Mask applies 1 Weak to all enemies at combat start, which is a 25 percent cut to everything incoming for the whole fight, and in co-op the debuff is shared across the entire team, so one mask weakens the boss for four players. Anchor's flat 10 Block at combat start is plain but always online, and Sai, 7 Block at the start of each turn with a 92.3 percent win rate on the community Ancient ladder, is exactly the recurring mitigation a stall deck wants from an Ancient blessing.
Energy relics matter more here than in most decks because your money cards cost real energy: End of Days at 3, Reaper Form at 3, and Scourge chains that want three casts a turn. Ectoplasm's flat plus 1 energy per turn is the strongest version, and the gold lock hurts less for this archetype than for shop-reliant ones, but weigh it against how many removals you still need. Lantern's first-turn energy accelerates Countdown or End of Days by a full turn, and Ice Cream lets you bank toward an End of Days turn instead of bleeding overflow. On draw, Bag of Preparation seeing two extra cards on turn one dramatically improves the odds Countdown is live early, and Gambling Chip's start-of-combat discard-and-redraw swaps dead Souls for Scourges. Fiddle is the caution case: drawing 2 extra at turn start but no more that turn directly fights your Soul cycling, so pass it when offered.
Two damage-adjacent picks overperform. Gremlin Horn, 1 energy and 1 card whenever an enemy dies, chains through Wrigglers and bees as your executes land. Mercury Hourglass deals 3 to all enemies every turn, free chip damage that shortens every stall turn while your attacks feed Reaper Form. Lizard Tail's once-per-run revive at half Max HP is pure insurance, and Strawberry and Pear patch the roster's lowest starting HP.
Counters, Failure Points and the Two Pivots
The hard counters are specific and worth writing down. Mecha Knight carries 3 Artifact on top of its 4-turn loop, and Artifact stacks absorb debuff applications, which means your first three Doom applications can simply vanish into it. The fix is to spend cheap debuffs first: Defy's Weak eats an Artifact stack, and Osty attacks apply no debuffs at all, so strip the stacks before committing Scourges. The same logic applies more broadly in co-op, where the scaling rules let Artifact, Slippery and Skittish stack higher with more players even after the v0.104 tuning pass. Test Subject #C10 is the structural counter: DoTs reset between its phases, so every phase is a fresh stacking race and phase 1 punishes the skills your engine runs on. Against C10, accept the long game, front-load Countdown early in each phase, and hold Osty resources for the skill-punishment window.
The most common failure point is not a counter, it is a stall. Doom is a late engine, and if your Act 1 rewards offered no damage and you took none, Byrdonis at 91 and Terror Eel at 140 HP will grind you down before the counter matters: the bestiary literally describes the Eel as a test of staying power. The fix is bridge damage, and it is why this guide tells you to take Reave and Poke early even though neither is a Doom card. The second failure mode is dilution: Countdown's random target across Entomancer's ever-growing bee board spreads stacks too thin to ever reach a threshold, so kill small bodies first and concentrate the ticks. The third is the Soul trap covered earlier, velocity without progress.
Pivot one is the Haunt Soul engine. If Soul density is high, Haunt plus Reave becomes a self-sufficient damage engine, 6 to 8 HP lost per Soul played, and the deck stops needing Doom as its primary clock. Our database rates it the S-tier Soul engine for a reason, and it carries a run on its own. Pivot two is the Reaper Form hybrid: with Reaper Form active, every Unleash and Poke and Reave swing converts its damage into Doom, and the deck's two resources merge into one. This is the strongest Act 3 shape available to the character.
When to abandon Doom entirely: if by mid Act 2 you have found neither Doom rare nor a second Countdown and your card rewards keep showing Soul and Osty support, ride the Haunt engine and treat Doom as a splash. The archetype's floor is a resilient midgame deck; its ceiling is the only percentage-based execute in the game, and in four-player co-op that ceiling is unmatched by anything else on the roster.