Darksteel Garrison is a rare Artifact — Fortification card from The List (PLST), collector number FUT-167. Current market price: $0.33, 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.)