Casualty is the mechanic associated with Maestros, which is represented by the color combination of Blue, Black, and Red. The color combo is also known as the Grixis shard from the Alara block.
The color combo in Streets of New Capenna is all about sacrifice and removal, and Casualty perfectly fits that theme.
Similar to Kicker, Casualty is an additional cost mechanic, but instead of paying mana, players pay by sacrificing a creature with power that is equal to or greater than a number that immediately follows the keyword “Casualty.” For instance, if a card says “Causality 1,” you would need to sacrifice a creature with at least one power.
“As you cast this spell, you may sacrifice a creature with power N or greater,” the reminder text of the mechanic reads. “When you do, copy that spell.”
By paying the Casualty cost, you can copy the spell that you are casting with Casualty. This mechanic was designed to be the domain of Instant and Sorcery spells. But there is one permanent (Ob Nixilis, the Adversary) that has Casualty.
There are 11 cards so far that have been spoiled that have Casualty, and all of them are one of the colors that comprise Maestros (Grixis). Five of them are Black and Four are Blue. One is Red and one is a combination of Red and Black.
This article will be updated once all of the Casualty cards have been revealed.