Gravelgill Duo is a common Creature — Merfolk Rogue Warrior card from Shadowmoor (SHM), collector number 165. Current market price: $0.11 (foil $0.35), TCGplayer market prices via Scryfall.
Whenever you cast a blue spell, this creature gets +1/+1 until end of turn. Whenever you cast a black spell, this creature gains fear until end of turn. (It can't be blocked except by artifact creatures and/or black creatures.)