You are a spirit bound to an object of power - perhaps a ring, tome, or crown. Alternatively, you might be bound to something more abstract, such as an office or line of descent, as long as this can be passed from one person to another. Many a “sorcerer” merely binds one such as you, until they meet their doom.
HD: d8
Save and spell progression as wizard/MU.
You must unerringly obey whomever bears your object of power, if they are mortal.
Level up when and only when your mortal master dies stupidly, wastefully, or tragically (allowing you to eat their soul.) You cannot act against them until they make a request of you.
If you die due to HP loss, or if your object is disenchanted, a lose a level and reform at the next full moon. If brought down from first level, permanently reform as an imp or quasit or equally silly little guy. If your fetish is destroyed, or if the bearer of the fetish dies either happily of old age or in genuinely valuable heroic sacrifice, begone for ever.

d10 (or lower for less weird) fetish
- ring
- sword
- tome
- crown
- lamp
- wand
- bloodline
- estate
- It Follows-style curse
- imp bottle
d6 spell list (or pick your favorite weird one)
- wizard/MU
- cleric (chaotic)
- illusionist
- necromancer
- cleric (lawful)
- druid