Gemini Engine is a rare Artifact Creature — Construct card from Darksteel (DST), collector number 121. Current market price: $0.26 (foil $0.71), TCGplayer market prices via Scryfall.
Whenever this creature attacks, create a colorless Construct artifact creature token named Twin that's attacking. Its power is equal to this creature's power and its toughness is equal to this creature's toughness. Sacrifice the token at end of combat.