Phantasmal Killer 4th-level Illusion
You tap into the nightmares of a creature you can see within range and create an illusion of its deepest fears, visible only to that creature. The target makes a Wisdom saving throw. On a failed save, the target takes 4d10 Psychic damage and has Disadvantage on ability checks and attack rolls for the duration. On a successful save, the target takes half as much damage, and the spell ends.
For the duration, the target makes a Wisdom saving throw at the end of each of its turns. On a failed save, it takes the Psychic damage again. On a successful save, the spell ends.
Using a Higher-Level Spell Slot
The damage increases by 1d10 for each spell slot level above 4.