The Eye of Truth, or Satan's Eye of Truth, is a tool that is used by the king of demons himself. It is classified as a third eye that is located on the Devil's forehead and characterised by a ripple-like pattern which spreads over the eye ball, with a red sclerae and irides, and a pentagram symbol etched on the iris itself.
The eye manifested shortly after Satan was cast down from Heaven alongside the angels and Lucifer himself that rebelled against God. While Satan is usually seen with two eyes by the denizens of Hell, he is interpreted as having three eyes and three heads in modern literature and past. However, it is revealed that the third eye is not merely a symbol, but an actual ability of Satan.
The Eye of Truth, once activated, allows Satan to be in total control of the environment and his surroundings. In his words, he becomes the "God of this World". Which is a nod towards one of his epithets as being the Prince of this World. When activated, Satan becomes the master of reality itself and can perform various abilities with this. He can turn back or speed up time, be it a millisecond or even a century. He is also capable of manipulating the minds of others, and forcing them to see what he wishes them to see, however he deems fit.
According to Lucifer, the Eye of Truth allows Satan to "split the fabric of reality", which basically means he is capable of tearing the boundries between the human world, and the supernatural world. Basically the 'lies' that cover the truth from all.
Satan, however, only activates this power when he is faced with an opponent that is worthy and one that rivals him in terms of power. Many demons stated that it is rare for their King to use this technique, but once activated, his opponents do not live to see it a second time. It is said that the Eye is also a way for Satan to manipulate humans, turning them away from God and their faith.