Diamond Knight is a uncommon Artifact Creature — Knight card from Core Set 2020 (M20), collector number 224. Current market price: $0.30 (foil $0.45), TCGplayer market prices via Scryfall.
Vigilance (Attacking doesn't cause this creature to tap.) As this creature enters, choose a color. Whenever you cast a spell of the chosen color, put a +1/+1 counter on this creature.