Team up with 2–4 players: enemy HP scales with party size, and coordination matters more than raw power.
Co-op supports 2–4 players, Steam friends only — there is no public matchmaking or lobby system. The host creates the room from the multiplayer menu and invites via the Steam overlay, and must stay online for the whole session. Saves can be resumed, but only with the original roster, and you can't run multiple campaigns at once. Ascension uses the party's lowest level and progresses separately from solo. All players must run identical mods in identical load order or the game desyncs — safest is to disable mods entirely. PC only (Windows/Mac/Linux/Steam Deck).
Everyone plays at the same time, enemies attack ALL players at once, and Vulnerable/Weak are shared across the whole team — one player applying Vulnerable multiplies everyone's damage. Relics drop in player-count batches and are drafted one each (ties resolve via rock-paper-scissors); shops are per-player. At campfires the new Mend option heals another player for 30% of their Max HP. Enable Fast Mode in settings to keep four-player animations from dragging.
There is no free trading system — cards move between players only through specific co-op cards. The Ball is a colorless 1-cost 10-damage attack that passes to a random teammate and gains +15 damage each time it's passed. Soulbound (Necrobinder) links a teammate so your Souls enter their deck too. v0.108 added 15 co-op cards, including Midnight, Blaze, Blade Symphony, Concoct, Plot, Constellation, Underworld, Hibernate and One For All.
A player who falls in combat revives at 1 HP on the next floor as long as at least one teammate survives; a full wipe ends the run. 'Keep the tankiest player alive' is the iron rule — funnel defensive potions to them when the team's HP runs low, and use campfire Mend to lift a revived ally off 1 HP. Note that Fairy in a Bottle still only saves its owner.
Per the official wiki, co-op HP = base HP × player count × an act coefficient (×1.1 in Act 1, ×1.2 in Act 2/3, ×1.3 for the Act 3 boss) — so 4 players face roughly ×4.4–5.2 HP, not the ×1.75/2.5/3.25 figures sometimes quoted. Enemy attack damage does NOT scale with player count, but Plating, Artifact, Slippery and Skittish stack higher, and enemy Block also scales. This is why percent-based Doom executes (Necrobinder) are so prized in co-op.
Community-consensus roles: Ironclad main tank, Silent debuff/poison amplifier, Defect flex (Frost Block or Lightning damage), Regent burst DPS banking Stars across turns, Necrobinder board control whose Osty soaks hits for the team. Classic lineups: Ironclad + Silent for duos; Ironclad + Silent + Regent for trios; Ironclad + Silent + Defect + Regent for four. Avoid duplicate characters — they compete for the same card and relic pools.
