Undercroft: Deckbuilder Roguelike Bosses Guide: Confirmed Bosses and the Rat King Fight

Undercroft: Deckbuilder Roguelike doesn’t hand players a boss list up front, but between the developer’s own listing and recorded gameplay, a real picture of the game’s bosses exists. Here is what can be confirmed, and what the one boss fight on camera — the Rat King — actually shows about how these battles play out.

Quick answer

The official listing for Undercroft: Deckbuilder Roguelike states there are 30 bosses guarding the way down, and that every descent ends with something waiting at the bottom. The only boss individually confirmed by observed gameplay is the Rat King: a hard-hitting, multi-turn fight that the recorded run won with guard cards, steady attacks and potions.

• The 30-boss count comes from the developer’s official description, which does not name the bosses or give an encounter order.
• The Rat King is the one boss named in observed gameplay, revealed immediately after a merchant stop and an upgrade-a-card node.
• In the recorded fight, guard cards absorbed the big hits, attack cards kept up the pressure, and potions carried the player through — the win came after the last potions were spent.

Which bosses are confirmed

The clearest confirmation comes from the developer itself. The official experience description promises “30 bosses guarding the way down,” and explains the shape of a run in the same breath: every descent is different, and every one ends with “something waiting at the bottom.” Bosses, in other words, guard the way down and wait at the end of each descent — guaranteed content, with variety in what specifically shows up to fill that slot.

What the listing does not do is name those bosses. There is no official roster, no encounter order and no checklist of who waits at the bottom of any given run. That makes observed gameplay the other way to confirm individual bosses — and in the gameplay on record, exactly one boss is called out by name: the Rat King, announced in a recorded run with the words “we’re fighting the rat king.”

The confirmed picture therefore has two layers: 30 bosses confirmed as the game’s design, and one of them — the Rat King — confirmed as a named encounter players actually fight.

How the recorded run reached the Rat King

The Rat King did not appear out of nowhere. In the recorded run, the reveal came at the end of a stretch of completely normal run activity, immediately before the battle began:

Mechanic flow

1. 1

   Merchant stop

   The run reached a merchant, and the player stopped to shop before moving on toward the next round.

2. 2

   Upgrade a card

   An upgrade node followed. The player upgraded a damage card, and the upgrade dropped the card’s cost to zero.

3. 3

   The reveal

   Immediately before the battle, the boss was announced: “we’re fighting the rat king.”

4. 4

   The fight

   A multi-turn battle against a boss that hits very hard, won after the last of the potions were spent.

Shopping, upgrading, then a boss reveal — that is how the recorded example funneled into its bottom-of-the-run fight.

How the recorded Rat King fight played out

The narrator’s commentary makes the boss’s character clear fast: the Rat King hits very hard. The recorded response was a turn-by-turn mix rather than any single trick:

• Guard cards came out on the dangerous turns, with back-to-back guard plays appearing more than once.
• Attack cards kept the pressure on between the big swings — slash, vendetta, rebuke, set jaw and flanking slash all saw play.
• Potions were the safety net. “I have potions so he literally can’t die” is how the narrator put it mid-fight, and the fight bore that out: when the damage got scary, a potion went down.
• Continued attacks across turns — the stated plan was simply to keep slashing him to death, and the player stuck to it.

The exchange lasted multiple turns, and the boss’s damage stayed punishing right to the end. It closed the way the safety net promised: the remaining potions got used, a final vendetta went out, and the fight was won.

One recorded fight can’t say everything, though. It does not reveal the Rat King’s stats, a phase system or a required strategy — it shows one real fight, won with the tools that run had on hand.

The tools in hand when the boss arrived

The same recording shows what the player brought into the Rat King fight, observed through the run’s earlier battles, including an Elite fight immediately before the boss segment:

• Attacks such as Slash, Kick the Fallen, Rebuke and Hammer Down.
• Guard and block cards for absorbing damage.
• Potions, used by tapping the potion.
• Card upgrades, which can reduce a card’s cost to zero — exactly what happened at the upgrade node before this boss.
• Stuns and Exposed are part of these fights: recorded attacks left enemies stunned, and cards applied Exposed to them.

Those systems are shared across every fight in the game, so the combat guide covers them in full, and each named card appears in the cards list. For shaping the deck you will actually carry into a boss, start with the deckbuilding guide.

What this means for your descent

Every run ends with something waiting at the bottom, and with 30 of those encounters in the game, most remain unobserved and unnamed in shared gameplay. The practical reading for now: treat the bottom of the descent as a guaranteed encounter, and note that the one confirmed example hit hard enough that the winning run leaned on guards, steady attacks and potions held for the worst turns. How the runs and paths that lead down there fit together is covered in the runs guide.