Nathrivox, the Obsidian Plane

Credits to Calder Moore from Artstation for the dreadful cover art!

Realm of death, soul, and negative energies, Nathrivox is the Elemental Realm of Darkness and Death. Dubbed the Obsidian Plane due to its usual (and sadly correct, post-Reckoning) association with the Obsidian Lords, Nathrivox is a necessary component of Yophas as a counterbalance to the light of Iannathae and Yacrena. Before the Reckoning, Nathrivox was a peaceful, if not eerily silent realm, whose dark black monoliths channeled the necessary death and soul energies to maintain the cycle of life and death across the Eternal Design. Today, the Obsidian Plane still fulfills its functions -- thankfully -- but a great many remnants and ruins of the Reckoning remain.
Nathrivox's border region is a realm of decay and death known as Xaethra. Once, its shadowed, sloping hills were utterly silent, a silence broken only by the crackling of energies flowing from the Primordial Dark and through the onyx black monoliths that dot the landscape. But in the Reckoning, Xael, the Witherer and god of entropy and void, came to Nathrivox and feasted on the Primordial Dark and the soul energies of the plane. The monoliths of Xaethra were forever scarred, becoming warped and ephemeral as the Witherer wore away at the very foundations of the Obsidian Plane. Today, these monoliths still stand over a landscape scarred by war, marred by decay, and crawling with undead and other, even more vile survivors of the Obsidian Lords' forces.
Beyond Xaethra lie the Mistlands, a realm of prison planes created by Alindr and Yamma as a way to keep non-divine malefactors away from the rest of the Grand Design. Rumors of the Mistlands abound, but many people believe that they have prisoners from across all of Yophas, and even from beyond the furthest known reaches of Xeccamund, and that these people are kept in demiplane prisons that parody their lives and homes and torment them unto eternity's end. 
And beyond even these, where the Mists grow too dark and cold for any entity, living and otherwise, to exist, is the Primordial Dark. Source of all darkness, death, and soul energy in Yophas, the Primordial Dark is a realm where nothing exists save for the dark. More than a void but less than anything real, this place is a realm of pure, unending energy that withers and consumes anything that has the misfortune of entering it. Like the Primordial Light in Iannathae, the Primordial Dark is channeled into the monoliths of Xaethra and dispersed across Yophas so as to maintain the balance of life and death, and darkness and light.

Inhabitants

Within Xaethra, there are few creatures of import and even fewer -- which is to say, no -- creatures of good intent. It is a barren waste of decay and death, wandered by the confused or starving survivors of ancient and broken armies. Infernal, undead, aberrant, it matters not. They have little in the way of purpose and less in the way of hopes of escape, and so they wander, eager to pounce upon anything or anyone that they stumble upon. In the Mistlands, almost anything is possible. The inhabitants are as wildly varied as the prison planes they live within (for an idea of what I mean, read Von Richten's Guide to Ravenloft), and in the Primordial Dark, there is nothing.

Type

Dimensional plane