The Darkness Crystal is a rare Legendary Artifact card from Final Fantasy Promos (PFIN), collector number 96s. Current market price: — (foil $10.76), TCGplayer market prices via Scryfall.
Black spells you cast cost {1} less to cast. If a nontoken creature an opponent controls would die, instead exile it and you gain 2 life. {4}{B}{B}, {T}: Put target creature card exiled with The Darkness Crystal onto the battlefield tapped under your control with two additional +1/+1 counters on it.