Call of Oblivion

 You use negative energy to attempt to sap the life essence of one creature that you target with this spell.

 The target of this spell makes a constitution saving throw against your spell DC.  On a successful save nothing happens. On a failed save the target gains one level of exhaustion.  At the beginning of every subsequent turn of the target the target repeats the constitution saving throw on a failure the target gains another level of exhaustion, on a success the spell ends and the target does not gain another level of exhaustion but all current levels of exhaustion that the target has stay on it until it completes a long rest. This spell has no effect on undead or constructs.