Border Guardian is a uncommon Artifact Creature — Knight card from Unstable (UST), collector number 140. Current market price: $0.10 (foil $0.26), TCGplayer market prices via Scryfall.
Whenever you cast a silver-bordered spell, put a +1/+1 counter on this creature. Whenever you cast a black-bordered spell, this creature can't be blocked this turn. Whenever you cast a white-bordered spell, this creature gains double strike until end of turn.