Casting summons a green, disembodied, floating eye that can move around while floating a yard or so from the ground that gives the caster a view from the "eye". While the "eye" is active, the caster is effectively in a channeling trance, so any interruption will cancel the "eye". The eye only has a single point of health, so any attack will easily destroy it. 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.
While previously the Eye was stealthy, this feature has been removed. Previously the spell description reads "The eye is stealthy and quick, but very fragile." Now, it simply reads "The eye moves quickly but is very fragile."
Most mobs will not attack the eye unless it's used to attack them first. Due to its low health, any mob will one shot the eye. If you use it to attack an instance boss, the boss may do some of their custom emotes used when you start a boss encounter, but the eye getting killed does not seem to aggro the boss onto the players. One notable exception is Blackheart the Inciter in Shadow Labyrinth. If you use the Eye on him, he will stay aggroed on the part after the eye is dead, and also pull the entire room of mobs he is in after the party as well. Using the Eye to scout bosses is handy in Serpentshrine Cavern to clear a roaming Murloc pack from the tunnel leading to him. Use the Eye to go up and attack the boss, and the mobs despawn, the same way they would on a normal boss attempt, but the eye attack in this instance does not aggro anything onto the party.
As of lately, it appears the Eye has more health and can attack. You can use it for tricky pulls.
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.
Eye of Kilrogg has a less chance to aggro mobs, making it even more useful in instances.
The eye also has a slow fall effect on it (since 2.0) allowing the warlock to view farther than before.
The [Glyph of Eye of Kilrogg] increases the movement speed of the eye by 50% and allows it to fly in areas where flying mounts are enabled.
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.
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.