The 5e feat of elven mage-slayer means you have mastered melee combat techniques that are not against spellcasters. These are some of the benefits you can enjoy.
- You can cast spells when a creature is within 5ft. You can also use your reaction against the creature to make a melee weapons strike.
- If you deal damage while a creature is concentrating on the spell. This occurs when a creature is unable to focus on the spell and suffers a disadvantage on its saving throw.
- You are able to make saving throws within 5 feet of your body, which is an advantage.