Slay the Spire is a 2019 roguelike deck-building game developed by the American indie studio Mega Crit and published by Humble Bundle.

Each opponent on the field will telegraph what move they will make: if they will attack and with how much damage, if they will block, cast a spell to buff themselves or debuff the player.

Otherwise, if the player defeats all the monsters in an encounter, they typically receive gold and a choice of one of three randomly-selected cards to add to their deck, if desired.

Other loot that can be obtained from monsters or other encounters include relics that provide a permanent character buff for the duration of the game, such as increased maximum health, additional energy points, lower casting costs, and automatic blocking, and single-use potions that can be used during any turn for no cost to restore health, buff the player, and debuff or damage the enemy.

Up to 20 Ascension difficulty levels unlock with each successfully completed run, each adding a cumulative negative effect such as lower health or stronger enemy attacks.

Achievements provide several challenges to the player such as to climb the tower with a character using only their starting relic or with only common cards in their decks.

[9] Originally, enemies did not show their next intended action as is common to most turn-based role-playing video games, but this design did not mesh well with the roguelike nature of permadeath.

[7] During playtests, they found that players were confused about the number of card abilities without any clear situation to apply them.

[11] They first created a "Next Turn" system where the player could select individual targets to see what their chosen action would be in part of the game's user interface.

The "Next Turn" system brought something unique to the game, according to Giovannetti, and enabled them to create new buffs and debuffs that were easy to describe to players through the interface.

[7] The approach for players to progress through the spires with branching pathways and random events was inspired by FTL: Faster Than Light.

[16][17] An official fourth playable character was added to the game as part of a free content update, first released to Windows versions on January 14, 2020.

It was not until a Chinese streamer featured the game on their channel that had over a million views, did Mega Crit start to see the large rise in sales.

[28] IGN named Slay the Spire as the Best Strategy Game of 2019,[46] while PC Gamer gave it the Best Design for 2019.

[52] A crowdfunding campaign for the board game was launched on November 2, 2022, and reached its funding goal after just 6 minutes.

[55] Mega Crit announced in April 2024 that a sequel, Slay the Spire II, was planned to be released in early access in 2025.