Traitor's Clutch is a common Instant card from Time Spiral (TSP), collector number 137. Current market price: $0.12 (foil $0.48), TCGplayer market prices via Scryfall.
Target creature gets +1/+0, becomes black, and gains shadow until end of turn. (It can block or be blocked by only creatures with shadow.) Flashback {1}{B} (You may cast this card from your graveyard for its flashback cost. Then exile it.)