Darksteel Sentinel is a uncommon Artifact Creature — Golem card from Scars of Mirrodin (SOM), collector number 152. Current market price: $0.30 (foil $1.71), TCGplayer market prices via Scryfall.
Flash (You may cast this spell any time you could cast an instant.) Vigilance Indestructible (Damage and effects that say "destroy" don't destroy this creature. If its toughness is 0 or less, it's still put into its owner's graveyard.)