Mage Slayer: Attack Caster is a passive feature provided by the Mage Slayer Feat. When a creature casts a spell near you, you can attack the caster as a Reaction.
Description
When a creature casts a spell within melee range of you, you can use a 
Reaction to immediately make an attack against the caster.
How to learn
Granted by the feats: Mage Slayer