Mage Slayer

Mage slayers are trained to sniff out mages on the battlefield and then eliminate them when possible, training their bodies and minds in order to resist the spells that would come their way should they fail to assassinate their target immediately. Even when a mage tries to run, it is already too late for them as mage slayers have learned how to track even the most persistent of those who try an escape.

Antimagic Sleuth

At 3rd level, you gain advantage on all Intelligence (Investigation), and Wisdom (Survival) that have to do with creatures that can cast spells. Furthermore if you talk to a creature for 1 minute, you can detect whether or not the creature has the ability to cast spells and one of the highest-level spells it can cast at random if you succeed an opposed Intelligence (Investigation) ability check against the creature's Charisma (Deception) ability check. 

Still Here

At 3rd level, as a reaction when an adjacent creature moves away from you, you can move up to your speed directly toward the creature that moved away from you. This movement does not provoke opportunity attacks.

Magical Defense

At 9th level, you learn how to concentrate on the magical formulae that makes magic possible in order to dampen its effects on you. As a reaction when a spell that requires a saving throw is cast on you, you gain advantage to that saving throw. You can use this ability a number of times equal to your Intelligence modifier. You regain all expended uses when you complete a long rest.

Antimagic Strike

At 13th level, as a reaction to when a creature within 60 feet casts a spell, you can make a melee or ranged weapon attack against the creature with any weapon you have on your person. If you succeed on your attack roll, you damage the creature as normal and prevent the spell from being cast. The spell slot is expended as if the spell were cast. Once you have used this ability, you must complete a short or long rest before doing so again.

Magic Killer

At 17th level, once per turn when you successfully attack a creature that has the ability to cast spells, you automatically deal damage from your Sneak Attack ability, whether or not you reach the prerequisites to do so.