The Lord of Pain is a mythic Legendary Creature — Human Assassin card from Duskmourn: House of Horror Commander (DSC), collector number 3. Current market price: $10.66 (foil $12.71), TCGplayer market prices via Scryfall.
Menace Your opponents can't gain life. Whenever a player casts their first spell each turn, choose another target player. The Lord of Pain deals damage equal to that spell's mana value to the chosen player.