Frostwalk Bastion is a uncommon Snow Land card from Modern Horizons (MH1), collector number 240. Current market price: $0.14 (foil $1.39), TCGplayer market prices via Scryfall.
{T}: Add {C}. {1}{S}: Until end of turn, this land becomes a 2/3 Construct artifact creature. It's still a land. ({S} can be paid with one mana from a snow source.) Whenever this land deals combat damage to a creature, tap that creature and it doesn't untap during its controller's next untap step.