Ashpit is the plane of Oblivion that is associated with the Daedric Prince Malacath. The Ashpit consists of nothing but dust, ash and smoke; there is no ground, sky or air. It is one of the hardest realms to reach, except the ones that are completely inaccessible to mortals and when it is reached levitation and magical breathing is an absolute necessity.
As described in The Doors of Oblivion, you can see from one end of the world to the other in a million shades of grey. There are Palaces of Smoke, and certain vaporious creatures reside in the realm.
According to Sheogorath, the 'backbone' of Malacath's Realm (that is, the thing that keeps a great land together, such as the people of a kingdom) is an actual spine.
"...where anguish, betrayal, and broken promises like ash filled the bitter air."