Darksteel Garrison is a rare Artifact — Fortification card from Future Sight (FUT), collector number 167. Current market price: $0.59 (foil $11.11), TCGplayer market prices via Scryfall.
Fortified land has indestructible. Whenever fortified land becomes tapped, target creature gets +1/+1 until end of turn. Fortify {3} ({3}: Attach to target land you control. Fortify only as a sorcery. This card enters unattached and stays on the battlefield if the land leaves.)