Backfire

Choose a creature you can see within range. The next time it casts a spell, it must make a Wisdom saving throw. On a failure, it takes Necrotic damage equal to 1d10 times the spell's level, and the spell fails (the spell slot is still consumed), and the spell ends. On a success, it takes only half as much damage and the spell does not fail. A creature is unaware it has been affected by magic, both before and after attempting to cast a spell, unless it either succeeds on an Intelligence (Arcana) check vs your spell save DC, or has some way of sensing magic, such as the detect magic spell.