If a Monster Slayer Ranger has Enervation cast on her, chooses to use her Slayer's counter ability, deals damage, breaks the spellcaster's concentration, does she still take the 2d8 damage from passing the save even though the spellcaster is no longer concentrating on the spell?
The Monster Slayer Ranger subclass gets the Slayer's Counter ability.
At 15th level, you gain the ability to counter attack when your prey tries to sabotage you. If the target of your Slayer's Prey forces you to make a saving throw, you can use your reaction to make one weapon attack against the quarry. You make this attack immediately before making the saving throw. If your attack hits, your save automatically succeeds, in addition to the attack's normal effects.
The Text of the spell Enervation is as follows.
Duration: Concentration, up to 1 minute
A tendril of inky darkness reaches out from you, touching a creature you can see within range to drain life from it. The target must make a Dexterity saving throw. On a successful save, the target takes 2d8 necrotic damage, and the spell ends. On a failed save, the target takes 4d8 necrotic damage, and until the spell ends, you can use your action on each of your turns to automatically deal 4d8 necrotic damage to the target. (...)