Grimagia is a card game where players take the role of "Grimeisters", summoners who capture monster girls using magical books called grimoires. Players win by capturing 3 of their opponents' monsters, or through eliminating each other player by reducing their HP to 0.
Players share the same 100 card deck. Each turn, a player may perform three actions: playing a monster from their hand, performing a union (combining two monsters to make one stronger monster), and capturing another player's monster using a grimoire (which works in a rock-paper-scissors fashion depending on the opposing monster's type). There are also relic cards with various effects which may be played at any time.
Afterwards, monsters may attack the other players or each other. All monsters have different stats such as health, strength, defense, and special abilities. Monsters who attack become Rested and are defenseless against attacks by other monsters until the player's next turn.