Dark Impostor is a rare Creature — Vampire Assassin card from Avacyn Restored (AVR), collector number 92. Current market price: $0.37 (foil $3.70), TCGplayer market prices via Scryfall.
{4}{B}{B}: Exile target creature and put a +1/+1 counter on this creature. This creature has all activated abilities of all creature cards exiled with it.