Dack's Duplicate is a rare Creature — Shapeshifter card from Conspiracy (CNS), collector number 43. Current market price: $0.31 (foil $1.23), TCGplayer market prices via Scryfall.
You may have this creature enter as a copy of any creature on the battlefield, except it has haste and dethrone. (Whenever it attacks the player with the most life or tied for most life, put a +1/+1 counter on it.)