Yahenni, Undying Partisan is a rare Legendary Creature — Aetherborn Vampire card from The Lost Caverns of Ixalan Commander (LCC), collector number 214. Current market price: $0.37, 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.