Eye of Kilrogg is a core warlock ability learned at level 22. Casting summons a green, disembodied floating eye. It can move around while floating a yard or so from the ground, permitting the caster to see through it. While the eye is active, the caster is effectively in a channeling trance, so any interruption will cancel the eye. The eye has a slow fall effect so it will not take any type of fall damage. Tests suggest that the eye moves at approximately 200% of normal running speed, giving the eye an effective range of around 500 yards.
Strangely enough, the Eye of Kilrogg is immune to damage caused by lava, and can be used somewhat effectively to help find players who may have died while submerged in lava.
If you are using the Scrolling Combat Text feature and are being damaged while in vision of the Eye of Kilrogg, you will see the damage scroll from the eye. This is a useful way to know to cancel the Eye and take care of the nuisance.
If your regular minion becomes stuck, you can summon the Eye (despawning your minion); cancel the Eye, and your minion will appear at your side.
This spell was first seen in Warcraft II: Tides of Darkness. It was used by an ogre mage pretty much the same as it is now in WoW, except that it had a much greater speed, and the ability to fly (only ranged units could attack it). It was also possible to use it to have enemy computer controlled vessels destroy each other with friendly fire.