Mystic Forge is a mythic Artifact card from The Brothers' War Retro Artifacts (BRR), collector number 36. Current market price: $5.20 (foil $9.80), TCGplayer market prices via Scryfall.
You may look at the top card of your library any time. You may cast artifact spells and colorless spells from the top of your library. {T}, Pay 1 life: Exile the top card of your library.