Spellbreaker

You gain the following benefits.

Ability Score Increase. Increase your Strength or Dexterity score by 1, to a maximum of 20.

Spell Interception. When you make an opportunity attack (triggered by Mage Slayer) against a creature casting a spell, and the attack hits, that creature must succeed on a Constitution saving throw (DC = 8 + your proficiency bonus + the ability modifier used for the attack) or the spell is intercepted — it fails, has no effect, and its spell slot is expended as normal. On a failed save, the creature also takes 1d8 damage of the spell's damage type (or force damage if the spell deals no damage).