You are well versed in the tactics needed to bring down hostile spell casters. You gain the following benefits:

High Dexterity. Your dexterity score increases by 1.

Blind Spot. You gain a +2 bonus to attack rolls against spellcasters concentrating on a spell or spell-like effect. 

Distraction. Casters concentrating on a spell or spell-like effect have disadvantage on the saving throw to maintain concentration when you damage them.

Killing Blow. Once per long rest, when you are targeted by a spell, cantrip or spell-like effect in melee range to the caster, you can counter with a weapon attack as a reaction. If you hit, it is considered a critical hit.