Mishra's Self-Replicator is a rare Artifact Creature — Assembly-Worker card from The List (PLST), collector number DOM-223. Current market price: $0.36, TCGplayer market prices via Scryfall.
Whenever you cast a historic spell, you may pay {1}. If you do, create a token that's a copy of this creature. (Artifacts, legendaries, and Sagas are historic.)