Ikiryo

This minor spirit springs from the intense envy or hatred a living human or humanoid feels toward one person. Monsters cannot spawn one, nobody can create one deliberately, and its creator almost never knows it exists. Once formed it has its own will: its source cannot control or cancel it, and it never acts on that person's behalf.

It is pure psychic presence with no form at all, so detect invisibility and detect life miss it. Detect evil shows an evil aura around its victim, and detect harmony senses that something about the victim's situation is off without revealing the spirit. Only true sight or the like shows it, as a shimmering phantom of its creator.

It heads unerringly at full speed toward its target, wherever in the world the target is. If the target is on another plane when the spirit forms, it fades within a day unless the target comes back to the Prime Material in time. After first contact, it can follow its prey across any plane, staying until one of them is destroyed.

Each full day it remains, every ability score of the victim drops by 1, along with anything that relies on them; a wizard dropping from 10 to 9 Intelligence can no longer cast 5th-level spells. Unless it has been detected, the victim may only feel restless, weary and stifled, never grasping the scale of the loss. A score reaching 0 kills the victim and the spirit vanishes. Survivors recover nothing until it is banished or destroyed, then regain a point in each ability daily.

Protection from evil, 10' radius holds it off without harming it. Invisibility to spirits hides the target only while it lasts. Summon spirit forces it into its creator's exact form, strengths and weaknesses included, and destroying that form dispels it. A successful exorcism banishes it for good. Easiest of all is to find its creator: confronting that person with the spirit's existence ends it forever, whether or not they accept the blame.

Ikiryo have no lairs, treasure or groups. Any PC or NPC may spawn one or be its prey, as the DM decides; an NPC's hidden grudge against a PC is a natural seed.

Simple villagers shun anyone an ikiryo has attacked, believing such people bring misfortune.