Arcane Backlash (Wyrdsunder)

Reaction — Arcane Backlash. When a creature within 5 feet of you casts a spell, you can use your reaction to strike them with Wyrdsunder. Make a melee weapon attack with Wyrdsunder. On a hit, the target takes normal weapon damage plus 2d8 thunder damage, and must make a Constitution saving throw (DC 16). On a failed save, the spell is disrupted and wasted.