Thrashing Mudspawn is a uncommon Creature — Beast card from Onslaught (ONS), collector number 177. Current market price: $0.13 (foil $0.62), TCGplayer market prices via Scryfall.
Whenever this creature is dealt damage, you lose that much life. Morph {1}{B}{B} (You may cast this card face down as a 2/2 creature for {3}. Turn it face up any time for its morph cost.)