Fallen of Obsidian Twilight: Calix Sabinus

From publisher blurb:

Wings spread, maw agape, sheathed in leather and gold and spun from the blood of thousands stands Calix Sabinus, god-emperor of the undead lords, bringer of death, destroyer of angels, a power before the cataclysm, a tyrant after. There is nothing more dangerous on all of Abaddon than Calix Sabinus. Calix has killed thousands, enslaved millions and crushed the wills of all that live on the lands of terror called Abaddon. To see Calix Sabinus can easily mean death. To actually meet Calix Sabinus face to face means worst that that. Tempt fate if you dare.