The All-Seeing Eyes of the Gods (神々の義眼 Kamigami no Gigan?) is an artifact given to Leonardo Watch by Riga El Menuhyut in exchange for his sister's sight prior to the beginning of the series. Though largely a mystery, the eyes are a prime artifact, even to the creatures of the Alterworld, and are incredibly valuable. Thus, many people will attempt to obtain them, no matter what it takes.
Prior to Leonardo Watch, there have been at least 1000+ other confirmed records of the 'All-Seeing Eyes of the Gods' holders in both the human world and the Alterworld. In all instances of people receiving the eyes, the person closest to them has their own vision taken away. Riga El Menuhyut only appears to give the 'All-Seeing Eyes of the Gods' to humans when a major event occurs in the human world, such as the event of Hellsalem's Lot, in order to record them.
The holders are chosen as the observers of major world-changing events, willingly or otherwise, and a contract is made between the holder and Riga El Menuhyut. In exchange for the eyes, the person closest to the holder is blinded. This serves as both a sacrifice for the manifestation of a divine miracle and as an anchor to keep the holder of the eyes in check, so that they cannot act rashly and can only observe obediently in the stead of the celestial beings.
As it is the result of a contract with a celestial being, removal of the 'All-Seeing Eyes of the Gods' is not as simple as going through a surgical procedure, and the eyes cannot be cured or returned to what they were before. Doing so would be the equivalent of forfeiting the contract made, and a price would have to be paid for such an act.
Holders of the 'All-Seeing Eyes of the Gods' are able to see the 'The Truth' of the world. Possession of the eyes gives their wielders numerous abilities including heightened visual senses, allowing them to see fast movements normally impossible for human eyes to follow, or minute details with great clarity, and allows them to clearly gaze both the past and flawlessly predict the future. They are able to see past world altering illusions and perceive an individual's unique aura, including the scarlet winged aura of a Blood Breed. The eyes also reveal their "true name" if its heart is present.
Eye holders are capable of altering and controlling the vision of those around them, such as switching two people's field of vision, inducing nauseating hallucinations, projecting what the holder sees into the minds of others, and the power over visual illusions in general. The eyes also allow the holder to see through another individual's eyes and observe past events that they have witnessed. The eyes can also be used to track body parts that have been removed or extracted from a person even when it can not normally be traced by conventional means simply by viewing the aura of the person and matching it with said body part. The eyes are also equipped with a self-restoration function such that over time, they will heal themselves of any damage sustained.
The eyes do have their limits, however. Due to the huge amount of visual information that the owner's brain has to take in, it takes weeks for the holder to get used to using them after they are first implanted. Furthermore, prolonged use of the eyes' abilities causes them to heat up to the point where they will start burning the user. If the eyes are damaged enough, they will crack, making their powers unusable until they heal back again. If they're damaged to the point when they can't be repaired, then they will explode, killing their user.