I had an odd idea for a Warlock character who was pretty much under the control of his familiar. Backstory-wise, he had made a contract with an extraplanar entity that would allow it to live on the material plane in exchange for giving power to the warlock. However, the warlock edited his end of the pact so that the entity would lose most of its power and be bound to him and be dependent upon him, while the entity changed its end of the bargain to give it complete control over the warlock. End result: warlock gained his powers but lost his mind and will, while the entity found himself in control of the warlock, but bound to him and incapable of interacting with the physical world without his assistance (though he could see and hear in addition to being seen and heard).
Bottom line, warlock character with exotic familiar. Familiar is an extraplanar entity that lost its power and is only capable of doing things familiars are usually capable of. Familiar is also the actual character while the warlock is just a meat puppet the familiar needs to fight or do anything else significant.
My question is this: are there rules for/against this type of character, and what could be done to make it work while keeping the concept intact?