Desmond's Ray of Impairment

A thick beam of sickly dark green energy springs from the palm of your hand in the direction of the target creature.

On a hit the target takes 1d8 necrotic damage and must make a constitution saving throw against the casters spell save dc or gain one level of exhaustion. On a critictal hit the creature fails the save automatically. 

For the spells duration you can take the magic action to use the spells attack again. 

The exhaustion can be applied only three times to the same creature.

The exhaustion ends if the spell ends early or 10 minutes after being applied.

Up casting:

The spell gains one additional ray every second spell slot level above third. So two rays per action at level 5, and so on...