Improved Mage Slayer

You have learned how to execute spellcasters in melee combat, gaining the following benefits.

• When a creature within 5 feet of you is in the process of casting a spell, you can use your reaction to make a melee weapon attack against that creature before they finish their spell and the creature has to make a constitution check with disadvantage as if they were concentrating on a spell. On a fail, the creature’s spell fails and has no effect.
• When you damage a creature that is concentrating on a spell, that creature has disadvantage on the saving throw.
• You have advantage on saving throws against spells cast by creatures you can see.
• You have advantage on ending magical effects on you, and you have advantage on detecting magic effects.