Ancient Phantasmal Killer

You tap into the nightmares of a creature you can see within range and create an illusory manifestation of its deepest fears, visible only to the spell caster and that creature. The target creature becomes [condition/Frightened] of the Manifestation. The Manifestation has a movement rate of 5 higher than the target creature and attacks with 1d20+4 against the target creature. The Manifestation is invulnerable to all attacks and can pass through any barrier as it exists only in the target creature's mind. If the target is successfully struck, it dies from fear. The only defense against a phantasmal killer is an attempt to disbelieve, which can be tried but once, or slaying/rendering the caster unconscious. The target must make a 3d6 saving throw rolling a total equal to or lower than it's intelligence score. additional modifiers apply to the dice save roll:

Condition
Target unaware of caster 3d6+2
Target aware of caster 3d6+1
Target previously attacked by this spell 3d6-1
Target is School of Illusion 3d6-2
Target wearing a helm of telepathy or naturally telepathic 3d6-3 and the ability to turn the phantasmal killer upon the caster if disbelieved.

Note that resistance/immunity to being frightened applies to this spell and gives advantage/immunity against it. intelligence modifier applies to saving throws as well. Negative modifiers add difficulty, positive modifiers reduce difficulty.

At higher levels: Add a -1 to the spell for every level cast at level 5 and above.