Rend Magic

You spew forth pure magical energy down your arm before releasing it in a devastating attack. Make a melee spell attack against the target's AC. If it hits, the target takes 3d10 force damage, then takes ((rank ÷ 2, rounded up) − 1)d10 force damage each time it Casts a Spell for 1 minute; on a critical hit, double the initial damage and the damage when the target casts spells. This spell ends immediately if the target ends its turn without Casting a Spell.