The Eye of Sheol is a Gate hidden within the Boundary. All ways to access it are currently unknown, as the Novus Orbis Sequentia has kept extraordinarily silent over the matter ever since the end to The Third War of Ars Magus and its alleged connections to the Seithr phenomena, Black Corrosion and even the theorized Red.
In theory, the Gate is to take an individual to a sealed off part of the Boundary where they make contact with what's within, which is believed to be a Cauldron or Cauldrons. Though some scientists have theories that the Eye of Sheol is a Boundary in itself containing its own systems of power that mankind explored at some point. Ever since the end of the Third War of Ars Magus it's become supposedly impossible to gain access to what's within it, or even to explore that part of the Boundary.
Within the Eye of Sheol is known to be an extremely pure and deadly power, along with many other contents such as seithr, memories, and souls.
While no one has been known to access the Eye of Sheol itself since the time of the War, replica connections have been attempted in the desire to understand it and make contact with the objects inside it.
Very few know about the Eye of Sheol, such people being Nex who has glimpsed at it when he received his Grimoire hinting a possible connection to him and it. Sylar also seems to be aware of the Gate, along with Azure Sin Subjects.
The Eye of Sheol is not a commonly spoken element in current time. Most don't even know of its power or its existence, as it has been a subject heavily guarded by the Government's World Order Council.
The Eye of Sheol is also regarded to with warning by the Amanohokosaka Clan (EvoBlaze).