Yahenni, Undying Partisan is a uncommon Legendary Creature — Aetherborn Vampire card from Commander Masters (CMM), collector number 201. Current market price: $0.30 (foil $0.60), TCGplayer market prices via Scryfall.
Haste Whenever a creature an opponent controls dies, put a +1/+1 counter on Yahenni. Sacrifice another creature: Yahenni gains indestructible until end of turn.