Maddening Hex is a mythic Enchantment — Aura Curse card from Special Guests (SPG), collector number 70. Current market price: $5.10 (foil $7.43), TCGplayer market prices via Scryfall.
Enchant player Whenever enchanted player casts a noncreature spell, roll a d6. This Aura deals damage to that player equal to the result. Then attach this Aura to another one of your opponents chosen at random.