Possessed (3.5e Flaw)
From D&D Wiki
At any time the entity may make an attempt to influence the decisions and actions of the affected character and the character must make a DC10-30 will save (based on alignment with GM’s discretion) or be influenced without realising it is happening. Over time repeated failed rolls may change the characters outlook on the world and by conjunction their alignment; as the DC for a changed action is based off of the distance in alignment from the possessing entity this can be problematic.
Once daily the possessing entity will attempt to take full control of the affected character; this is done through opposing will rolls (as the possessing creature can be any extra planar the creature should not be more than 1 point higher in base will saves then the character).If the entity is in control it is the player who may attempt to influence the characters actions instead.