All Eyes is a seemingly omniscient being living in the caverns beneath Maretania. It has, at three or possibly more points in Equestrian history, imparted extremely powerful and dangerous arcane knowledge to ponies. In all known instances, the knowledge eventually led to violent upheaval.
It's name is derived from what it asks visitors to call it. When Lyra shines her spotlight into the rift, however, she was horrified to see the surface of its body entirely covered in moving eyeballs.
Theories on the nature of All Eyes, based on in-story dialogue and legends.
The most likely origin of All Eyes is cosmic. The legend from the Tribunal Hall says that there once were Six Ruler Stars of the northern sky. One star killed another, committing the universe's first murder, and was banished into the world for it. Afterward, only four "Ruler" stars remained. All Eyes hints, in the Coda section, that he may have once known those "bold, capricious stars".
Another possible origination is that it is the spirit of some primal force, like Discord. Candidate concepts could be a spiritual manifestation of Knowledge or even Magic.
The Planet ItselfEdit
Breakbeat speculates that All Eyes may be an avatar of the world itself, much like Princess Celestia and the sun or Princess Luna and the moon. This would explain his ability to see everything happening on the planet, as he is intrinsically connected to it.