Erdric always had a flair for the unusual. As a bargaining tool with a denizen from the lower planes, he created a form of scrying tool for them to use with a twist. The recipient of the spell becomes cursed and loses the sight and control of whichever eye the caster chooses at the time of casting. The enchanted eye then becomes in the control of a demon or devil of the lower planes which has been prearranged as in a "bargain" from the caster.
The eye moves in whichever direction the demon/devil wills it, regardless of what the targets other eye is doing. What's more, the target also loses 1d4 charisma due to the (creepy feeling) people get while interacting with the inflicted. A detect evil spell cast on the target radiates the evil of the creature possessing the eye effectively fooling any detection into sensing evil.
The spell can only be removed with a remove curse or the like spell. Higher castings of the spell only make the curse harder to remove, as it's assumed a more powerful denizen has bargained for it's control. High level lower planar creatures have been known to even use their own powers and influence through the use of this small connection to the material plane.
* - (Eye of any demon or devil)