Nexus of Becoming is a mythic Artifact card from The Big Score (BIG), collector number 90. Current market price: $3.45 (foil $4.50), TCGplayer market prices via Scryfall.
At the beginning of combat on your turn, draw a card. Then you may exile an artifact or creature card from your hand. If you do, create a token that's a copy of the exiled card, except it's a 3/3 Golem artifact creature in addition to its other types.