Only Auric Ascended gets the Avatar promotion, despite the name of the Avatar of Wrath and the fact that the Mithril Golem and Messhaber have been described as Avatar since at least 0.16. None of the archangels have the avatar promotion.
Meshabber is described as the ruler of the Iron City of Dis, which is found in Agares hell at the border of the regions where Agares' and Bhall's respective influences are dominant. He is presumably one of the strongest demon lords. If I had to guess, I'd say he was one of the strongest angels Bhall ever created (although not on Brigit's level), who fell with (or maybe even before) her and now wields great power of both fire and entropy, serving as a King over the Balors.
On the other hand, he looks a bit like a really big mane and the purpose of hell is to create not just ordinary demons but demon lords (it seems every leader but Hyborem and Basium in Lord of the Balors was once a mortal, although Balors themselves were not), so perhaps he is the most corrupted of all mortal souls. I kind of like the idea that angels and demons may be pretty much stuck at the level on which they were created while humans may the capacity to grow to become much greater. The human divine spark is on the level of a god's, and the experience a former mortal would have off all spheres could allow much more flexible and complex schemes.
Since Agares prefers competition among his subjects rather than a hierarchy (although he of course doesn't go so far as Camulos and prefer that all his servants try to kill him and usurp his throne), Meshabber probably has a great deal of autonomy. He likely views Hyborem as an equal, and only follows the command of Agares or Bhall.
