Undercroft Deckbuilder Roguelike Deckbuilding Guide: Add, Remove, Transform and Upgrade Cards
Undercroft Deckbuilder Roguelike deckbuilding guide: add, remove, transform and upgrade cards during a descent, and tell which offers are worth their HP cost.
Quick answer
While descending in Undercroft Deckbuilder Roguelike your deck keeps changing: winning a fight can offer a card copy, floor events offer card removal and transformation for HP, upgrade prompts improve one card of your choice, and a merchant sells cards and relics mid-run. Take what clearly serves your deck's plan, pay HP only to fix a card that is actively hurting you, and remember that declining any pick is allowed.
- Add — a fight reward reading 'add a copy of this card to your deck' puts that card straight into your deck; the merchant also sells cards and relics.
- Remove and transform — both were offered at one floor event at 6 HP each.
- Upgrade — prompts appear during a run and change a card's values; one upgrade in a knight run turned a card free.
- Every choice costs you — on one pick-some screen a player said “I don't think I'm going to pick any... Don't need to pick everything” and skipped the picks he didn't need.
Undercroft Deckbuilder Roguelike builds your deck inside the run rather than before it, and many deck changes are direct trades with an explicit price — the choice screens literally warn, “Choose carefully. Each choice costs you.” This guide walks through the four ways your deck changes during a descent, what each one cost in an observed run, and how to judge which offers are worth accepting.
Where each deck change happens
Each operation shows up at its own point in a descent, and one knight run contained all four:
Data table
| Operation | Where it appears | Observed cost | What it looked like |
|---|---|---|---|
| Add — fight reward | Immediately after winning a fight | None shown on the offer | The reward read 'add a copy of this card to your deck'; accepting placed the card directly into the deck |
| Add — merchant | A merchant node during the descent | A purchase (prices not shown) | Blood price, Press on, War cancel, Still disciplined and might cards were on offer, plus relics like Grave pipe |
| Remove | A floor-event choice screen | 6 HP | Listed in the same choice list as transform |
| Transform | A floor-event choice screen | 6 HP | Slash was transformed into Kick the Fallen |
| Upgrade | Mid-run — once after a fight, once after a merchant stop | None shown on the prompt | One upgrade turned a card free (zero cost) |
The 6 HP prices are what one event screen actually displayed, so treat them as observed examples rather than confirmed fixed rates. Each event states its own cost explicitly — read that line every time before you commit.
Adding cards: fight rewards and the merchant
You will meet adding at two points in a run: fight rewards and the merchant. Winning a fight can offer a card reward — in the observed run the offer read “add a copy of this card to your deck” — and accepting it puts that card directly into your deck. One such reward was taken purely for its draw power, which is the right lens for these offers: a fight reward is a yes-or-no choice, so the only question is whether the card helps your plan.
The merchant is the second source. Mid-descent, a merchant node appears where cards and relics can be bought; the observed offerings ranged from cards like Blood price and Still disciplined to might-related options, alongside relics such as Grave pipe. For a wider view of everything that exists, browse the cards list.
Removing and transforming at floor events
Floor events are where you actively reshape the deck. During a descent, an event node offered two card-manipulation choices side by side: remove a card for 6 HP and transform a card for 6 HP. Removing takes a card out of your deck, while transforming swaps one card for a different one — the player transformed Slash and it came back as Kick the Fallen.
These screens mix in other trades too. A separate floor choice offered gold in exchange for 4 max HP — a reminder that on these screens the price is paid in health, whether HP now or max HP. With “Each choice costs you” printed right on the screen, an event node is never a free stop.
Upgrading cards mid-run
Upgrade prompts appear during a run — in the observed run one showed up right after a fight and another after a merchant stop. You pick one of your own cards and the upgrade changes that card's values. Two results from the same run show the range: the player spent one upgrade on an attack card, and another upgrade turned a card free — a zero-cost card the player summed up as “zero cost is awesome.”
Compare before you commit. On one upgrade screen the player browsed his options, disliked at least one upgraded version on sight, and spent the upgrade somewhere else. The prompt improves one card at a time, so a quick pass over your deck first pays off.
Which choices are worth taking
Every deck offer passes through the same question: does what I get outweigh what the screen charges me? The costs are real — 6 HP per remove or transform in the observed event, 4 max HP for the gold trade — so your health bar is part of the price of deck improvement.
A few habits from that knight run are worth copying:
Pay for deck surgery when a card is actually hurting you. In combat, the player kept running into effects whose cost is paid in HP or block — a challenge card reading “Lose two block HP, gain 16 block” and a “Burn away” option priced at 6 HP — and commented that everything kept eating his health and block. The rest of his deck ran on cards like Slash, Rebuke, Hammer down, Set jaw, Vendetta and Blood price; the drain came from specific cost-bearing effects, not from every card. When particular effects chip you fight after fight, paying 6 HP to remove or transform the culprit starts to look cheap.
Read what a card gives the enemy, not just what it gives you. One Burn away card granted block and might but also handed might to the enemy, and the player rejected it on the spot as clearly bad. A tempting number on your side does not cancel out a gift to the other side.
Prefer changes that improve a card's economics. The zero-cost upgrade was the standout moment of the run — a card that plays for free pays off every time it lands in your hand.
Know when to walk away. Offered picks are optional: on one pick-some screen the player said “I don't think I'm going to pick any... Don't need to pick everything” and skipped the picks he didn't need. If nothing on a screen solves a real problem, declining keeps your health for the offers that will.
Decision guide
Where to take your deck next
Additions from fight rewards and the merchant build your plan, upgrades sharpen the cards you already lean on, and removes and transforms are paid surgery for the cards that hurt more than they help. Everything else — every event and pick screen — you are allowed to leave untouched.
For the bigger picture, the runs guide explains how a whole descent fits together, the combat guide covers how cards play out fight by fight, and the deck builds page assembles full decks. Still choosing who to descend with? The heroes guide covers your hero options.
Related Guides
Runs Guide
Follow the stages of a descent, from the first fight through the choices that move you down.
Combat Guide
Learn how a turn works: play attack and Guard cards, block hits, use free potions and win fights.
Deck Builds
Copy a documented Knight deck built around Expose and Kick the Fallen, Guard-backed attacks and free upgrades.
Heroes Guide
See the five advertised heroes, who is playable now, and when the Cleric update is scheduled to arrive.