Undercroft Deckbuilder Roguelike Deck Builds: A Documented Knight Run to Copy

Undercroft Deckbuilder Roguelike is a Roblox deckbuilder that one recorded player compared to Slay the Spire — similar in structure, though not an exact copy. The combinations below all come from that player's documented Knight run. Every card, effect text and fight sequence in this article comes from that recorded run, which ended in a win over the Rat King. It is one creator's early playthrough, not a verified best build, and no tier ranking is claimed — but each combination can be copied as-is or adapted into your own Knight decks.

Quick answer

The documented Knight run offers five copyable combinations: Expose effects feeding Kick the Fallen, an attack-plus-Guard shape on nearly every turn, upgrades that drop a card to zero cost, Reboot for draw power, and a survival layer built on a 'when you win a fight, heal six' relic plus potions saved for the Rat King.

• Expose + Kick the Fallen: apply Exposed with effects like 'apply three exposed', then cash it in on the card whose upgrade text reads '+ two of each exposed'.
• Attack + Guard turns: Slash, Kick the Fallen, Rebuke and Hammer Down were all paired with Guard while still attacking.
• Zero-cost upgrades: one card was upgraded until 'this just becomes free' — 'zero cost is awesome.'
• Reboot: taken from a card reward for its 'draw power' and played repeatedly through the mid-run fights.
• Rat King finisher: Steel Discipline, Flanking Slash, Vendetta and Rebuke as the late-fight chain, with every potion spent before the win.

The card pool the run played

The recorded Knight run used a card pool that included sixteen named cards: Slash, Guard, Reboot, Kick the Fallen, Rebuke, Hammer Down, Expose, Set Jaw, Blood Price, Vendetta, Steel Discipline, Flanking Slash, Iron Skinned, Armory, Ready for War and Double Hex. They were played as attack/block combinations each turn rather than one fixed combo. Exact effect text is documented for only a few of them — for the rest, what the run shows is when each card appeared, not its full card text.

Cards in the recorded Knight run

Slash

The workhorse attack, played from the opening fights through the Rat King finisher. Mid-run the player transformed Slash into Kick the Fallen, yet Slash kept appearing in later fights, including the final one.

Guard

The run's block staple. It was doubled up — 'double guard', 'guard guard' — when hits looked dangerous, and it appears in almost every recorded turn.

Expose

Marks enemies for bonus damage. Effect text observed in the run includes 'apply three exposed' and 'enemy loses might this turn'.

Kick the Fallen

The run's payoff attack, acquired by transforming Slash. Its upgrade text reads '+ two of each exposed', and the player weighed it as an attack when comparing upgrades.

Reboot

Chosen from a card reward for its 'draw power' and played repeatedly through the middle of the run.

Rebuke

Played constantly next to Guard — 'Rebuke. Guard' is the run's most repeated pairing — and it appears in the Rat King finisher chain.

Hammer Down

Played alongside Guard and Kick the Fallen in the mid-run fights.

Set Jaw

Held for the Rat King fight, where it was played next to Vendetta; earlier in the run it was passed over at an upgrade prompt in favor of Blood Price.

Blood Price

Chosen over Set Jaw at an upgrade prompt, and also listed among the options at the run's merchant stop.

Vendetta

Played throughout the Rat King fight, including the final play that closed out the win.

Steel Discipline

Opened the late-fight attack chain against the Rat King: Steel Discipline into Flanking Slash, Vendetta and Rebuke.

Flanking Slash

Played immediately after Steel Discipline in the Rat King finisher.

Iron Skinned

Played in the same stretch as Hammer Down and Rebuke during a later fight.

Armory

Played alongside Double Hex mid-run and taken again at the start of the Rat King fight.

Ready for War

Part of the recorded pool; the transcript captures it being offered mid-fight and declined ('Ready for war? No'), with no clear effect text shown.

Double Hex

Played right after Armory in one mid-run fight.

Combination 1: Expose feeds Kick the Fallen

This is the run's clearest synergy and the one most worth copying first. Kick the Fallen entered the deck through a transform event that replaced Slash, and its upgrade text — "+ two of each exposed" — pays out directly against enemies marked by Expose. Expose itself marks enemies for bonus damage, and the Expose-side texts observed in the run included "apply three exposed" and "enemy loses might this turn."

The limit on this combo matters as much as the combo itself. When a reward offered to pile on even more Expose application, the player declined: "we don't need that much exposed." Once the payoff card is in place, extra Expose stops being worth a pick — and the same reward screen drew the broader comment that you don't need to take everything a reward offers.

Combination 2: attack plus Guard on nearly every turn

The run's default turn shape was an attack/block combination rather than all-in damage. Recorded turns include Kick the Fallen with Slash and Guard, Rebuke with Guard, and Hammer Down alongside Guard. When a hit looked dangerous, the player simply doubled up on Guard — "double guard", "guard guard" — without stopping the attacks in the same turns. Potions layered on top as needed, with the player noting they are free to use.

Combination 3: zero-cost upgrades and Reboot draw

Cost reduction was the upgrade this run valued most. At an upgrade event, the player improved a card until "this just becomes free", commenting "zero cost is awesome" — and an earlier pick had already been flagged as "that's a zero cost."

Draw came from Reboot, chosen from a card reward for its "draw power" and played repeatedly through the middle of the run. Fight rewards also surfaced a card whose text adds a copy of itself to your deck — one observed way the pool kept growing between fights.

Combination 4: the survival layer

Three pieces kept the Knight alive long enough to use any of the above. First, a relic reading "when you win a fight, heal six", which drew the reaction "Okay, I can actually take a hit." Second, a healing option available after fights — "you could heal." Third, potions held back instead of spent early. Two more relics joined during the run from chests: Soul Jar and Hunter's Tally. By the Rat King fight the layer was doing real work — "I have potions so he literally can't die" — and every potion the player had went into that fight before the win landed.

The Rat King finisher, as played

The run ended against the Rat King with every combination above working in one sequence. The recorded plays, in order:

1. Opening turn: Slash, tank, Expose.
2. Armory, Vendetta, Set Jaw, burning away a card, then another Expose while tanking.
3. Guard, Guard, Slash, with a potion used as damage piled up.
4. Steel Discipline, Flanking Slash, Vendetta, Rebuke — "keep slashing him to death."
5. Set Jaw, the burn-away activation, Slash, Guard, Guard, while absorbing heavy hits.
6. The last potion — "I just used all our potions here, bro" — then Vendetta closed it out: "There we go, we won."

Read as a template, the fight applies everything earlier in the run: Expose applied early, attacks paired with Guard throughout, the Steel Discipline → Flanking Slash → Vendetta → Rebuke chain as the late-fight damage, and the potion stockpile as the reason the Knight survived. Note that the win cost every potion — the deck finished with nothing left in reserve.

How to adapt the run to your own Knight

The run's combinations are only half the value; the decisions that built them transfer too. Each fork below is a choice the recorded player actually made:

Decision guide

Choose for your situation

A transform event offers to replace a basic attack like Slash

Consider trading it for a synergy payoff. This run turned Slash into Kick the Fallen — the card behind its signature combo.

An upgrade prompt can drop a card's cost to zero

Take it. The recorded player upgraded a card until 'this just becomes free' and called zero cost 'awesome'.

A card reward helps you but also strengthens the enemy

Pass. This player declined a burn-away card granting 'gain three block / gain three might' because its text also made 'the enemy gains two one might'.

The deck already has its Expose payoff

Stop adding Expose application — 'we don't need that much exposed' was the exact call in the run.

An upgrade prompt offers several cards at once

Compare before committing: the run skipped a Kick the Fallen upgrade preview the player disliked, passed on Set Jaw, and took Blood Price.

One stop the run touched but did not fully document: a merchant appeared mid-run where the player considered a relic called Grave Pipe alongside options named Blood Price, Press On, War Cancel, Steel Discipline and Might. Exact prices and effects were not captured, so treat it as a sign that merchants stock card and relic options mid-run — not as a shopping list. The relics that verifiably joined the deck came from chests: Soul Jar and Hunter's Tally, on top of the "when you win a fight, heal six" relic.

Where to take these combinations next

To go deeper on the systems behind these combinations, the deckbuilding guide covers how individual picks fit a run's plan, the cards list documents the full card set, and the heroes guide puts the Knight alongside the other characters. For the fights themselves, the combat guide covers the mechanics and the bosses guide covers the Rat King, while the runs guide walks through a full run from start to finish.