Darksteel Reactor is a rare Artifact card from Darksteel (DST), collector number 114. Current market price: $8.09 (foil $61.23), TCGplayer market prices via Scryfall.
Indestructible (Effects that say "destroy" don't destroy this artifact.) At the beginning of your upkeep, you may put a charge counter on this artifact. When this artifact has twenty or more charge counters on it, you win the game.