So the situation is a character with the mage slayer feat is standing next to an NPC who casts the invisibility spell.
Pertinent verbiage from the feat:
1. You attack before the spell is complete. It is a normal attack and after the attack the enemy turns invisible regardless of the outcome (I really don't think this one is supported by the rules because he did not "cast a spell" and trigger the reaction)
2. Your attack happens at the exact same time as the spell finishes. It is a normal attack and the enemy turns invisible regardless of the outcome of the attack (as long as he is not dead from the attack).
3. Your attack happens at the exact same time as the spell finishes. It is a normal attack and if it hits the enemy must make a concentration save with disadvantage or he loses concentration and does not turn invisible at all.
4. Your attack happens right after the spell completes, the attack is with disadvantage because the enemy is invisible. If you hit the enemy makes a concentration save with disadvantage.
I am leaning towards #2 above.
Pertinent verbiage from the feat:
- 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.
1. You attack before the spell is complete. It is a normal attack and after the attack the enemy turns invisible regardless of the outcome (I really don't think this one is supported by the rules because he did not "cast a spell" and trigger the reaction)
2. Your attack happens at the exact same time as the spell finishes. It is a normal attack and the enemy turns invisible regardless of the outcome of the attack (as long as he is not dead from the attack).
3. Your attack happens at the exact same time as the spell finishes. It is a normal attack and if it hits the enemy must make a concentration save with disadvantage or he loses concentration and does not turn invisible at all.
4. Your attack happens right after the spell completes, the attack is with disadvantage because the enemy is invisible. If you hit the enemy makes a concentration save with disadvantage.
I am leaning towards #2 above.