An orb, made of wheels within wheels, covered with eyes facing all directions.  This is how angels of the order of Thrones are described in  the bible. Now imagine we merge that description  with the Seraphim, adding six sets of wings. Seraphim also had eyes all around their bodies, even under the wings.

Now twist that few degrees to the left, and abandon all reason. For the wheels are tubes of writhing flesh, the eyes all different, opening and shutting, some weeping. And each of the six wings is made of tubes of flesh as well, each ending in 6 longer tubular tentacles, that extend from the wing as a bat's claw does from its own wing. Each of those tentacles bears a fanged and gnashing mouth. And deep within the writhing mass of tubes within wheel-like tubes that it is the horror's body, a larger mouth rests, like a black hole surrounded by teeth, a lamprey's mouth, but with the Abyss for a throat.

This abomination is called a Witness. They see all that transpires near them, and much that occurs far away. Their vision does not experience time or space the way we do, and there is good reason for that. They are only the eyes of something larger, peering into our reality from a place where neither time nor space hold any meaning. And we have no idea what that larger thing is. We likely cannot even conceive of it. But it is very likely that some poor souls saw these things and thought that they must be the eyes of God.

As if its aspect weren't terrible enough, the Witness bears a panoply of powers. Not only do its myriad eyes grant it true sight , piercing illusions and seeing that which is unseen, meeting the gaze of those eyes can drive one mad, though that may simply be rational response to the horror of seeing it at all. It is capable of engaging nearly any amount of foes at the same time, deploying its fanged mouths on elongating tentacles, far beyond what might be considered a reasonable distance. Like the mythical basilisk, it can poison creatures simply by looking at them, or turn them to stone as one of the Gorgon sisters might. (Or salt, or ash, or a pile of refuse...) It flies swiftly, unerringly,  or hovers effortlessly in place, seemingly unbound by such mortal concerns as gravity. It can transpose itself to any location it can see without crossing the space between, as if it is actually present in all those places at once, and merely chooses where to manifest. Its powerful psyche can reach into the mind of any who look upon it, forcing compliance with its will, or re-writing memories...or draining them entirely, leaving its victim an empty husk, alive but dead inside, merely an external vessel for the consciousness of the Witness, which seems to have no difficulty dividing its perspective and attention between all such thralls, and has full knowledge of their past and present, allowing it to carry on as if nothing has changed. Some of those it slays rise again, likewise vessels for its will, but much harder to kill, as they are already dead, mere echoes and memories of what they once were, either embodied or existing as a sort of psychic figment that still can kill. These revenants are referred to as Memoria. To a lesser extent, the Witness seems to be able to rewrite reality around it to match some other version, perhaps choosing the conditions that pertain in alternate worlds and transposing them to our own.

For those who think the Witness is an angel, well, they may be right, for angelos simply means messenger, and it is certainly an emissary from something. Something dreadful, powerful, and unknowable.

For many realities, it is rumored, a Witness is the last living (if it can be called so) thing to have seen that world before its demise. Sometimes, it is said, they are the cause.