Blood is a unit of Currency that can be used to play Beast cards, which are cards created by Leshy.
In Game[]
Almost any card can be sacrificed to obtain 1 Blood. If a creature is not a possible sacrifice, they will usually differ in appearance - such as having a dotted outline and a symbol with an X over a dagger in Act I or a darker background in Act II. Creatures with the Worthy Sacrifice Sigil are worth 3 Blood instead and can be used as sacrifices even when they ordinarily wouldn't.
You will only be prompted to sacrifice creatures when you attempt to play a card with a Blood cost of 1 or more. If there are not enough worthy cards to sacrifice to play a card, the game will prevent you being able to play it. Sacrificing removes a card from play, killing it and giving you 1 Bone. Creatures with the Many Lives Sigil, such as the Cat, will not die when sacrificed but will still give their Blood.