Mage Hunter

 As a reaction you Attack a Mage, that is Within melee Reach, For trying to cast a spell. Target makes a Constitution Saving throw, with Disadvantage, Versus your Athletics to see if they can still cast  a spell. the Disadvantage is Negated if the target takes the War Mage Feat. If user has Mage Slayer, Ability turns From a Reaction to An Opportunity attack.