Yahenni, Undying Partisan is a rare Legendary Creature — Aetherborn Vampire card from Aether Revolt Promos (PAER), collector number 74p. Current market price: $3.88 (foil $4.53), 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.