Source: Player's Handbook

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

• 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.