I just realized that opponents sacrificing their creatures counts towards the Kill X of your opponent's creatures quest.

Was in a match with my [[Shessra's Death Whisper]] Brawl deck, which is my go-to deck to complete the Kill X of your opponent's creatures quest. Opp scooped around turn four before I could kill anything, but not before sacrificing a skeleton with [[Ashnod, Flesh Mechanist]]. Back on the home screen, I was surprised to see the quest wheel had ticked up from zero. Opponents sacrificing their creatures counts as a "kill."

Partly, this quest wording has always bugged me* because in Magic there is no rule that defines what it means to "kill" a creature. This is an Arena-specific term, and now we know one more part of how it's defined.

*Also the fact that it's singular "opponent's" instead of plural "opponents'" but that's being nitpicky.