Maddening Hex is a rare Enchantment — Aura Curse card from Mystery Booster 2 (MB2), collector number 195. Current market price: $2.00 (foil $7.48), 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.