Mage Slayer Feat

You have practiced techniques useful in melee combat against spellcasters, gaining the following benefits:

Prerequisite: None

• When a creature within 5 feet of you casts a spell, you can use your reaction to make a melee weapon attack against that creature.
• When you damage a creature that is concentrating on a spell, that creature has disadvantage on the saving throw it makes to maintain its concentration.
• You have advantage on saving throws against spells cast by creatures within 5 feet of you.

Source: PHB'14, page 168