Emperor of Bones is a rare Creature — Skeleton Noble card from Modern Horizons 3 (MH3), collector number 453. Current market price: $3.50 (foil $5.93), TCGplayer market prices via Scryfall.
At the beginning of combat on your turn, exile up to one target card from a graveyard. {1}{B}: Adapt 2. Whenever one or more +1/+1 counters are put on this creature, put a creature card exiled with this creature onto the battlefield under your control with a finality counter on it. It gains haste. Sacrifice it at the beginning of the next end step.