Deca Slayer takes the gameplay from Decathlon and moves it to a fantasy environment in which players now need various skills to complete tasks.
In this card game, players compete to slay a series of ten monsters and grab the treasure the monsters leave behind. While beating a monster gives points and potential bonuses in future rounds, not beating it gives a player first dibs on getting the replacement adventurer cards, giving an advantage in future challenges.
Each monster has a different rule for beating it, and players bid adventurer cards in turn — sometimes face up, sometimes face down — to fight it. The rules might be the highest card in a certain suit, or most cards that have a certain skill, or the sum of all cards the player has.
In the end, the player who slays the final dragon receives bonus points, then everyone sums their loot and points, with the player who has the highest total winning.