Tropos

By all the laws of divine ontology, there should not be a divinity whose province is the abomination of undeath. Gods can be cruel, destructive, even evil by any mortal standard, but the nature of divinity should rail against a disharmony like undeath at the most fundamental level.
The nature of divinity reckoned without the morbid rage of a Primordial Dragon.
Tropos was known as the Shadow of Death, the Grim Primordial. E was responsible, in the time before time, for shaping the gift of mortality which the Primordials settled on Orthos the Bound. Perhaps as a result, e grew sullen, sinister and insular, draw­ing the suspicions of Hroun and Arabast, precipitating the Schism which destroyed Ladon.
Slain by Arabast, Tropos fell towards The Abyss, but instead of completing that descent, e somehow drew in a massive amount of Necrosis which fused with eir remaining divinity to create something new and terrible.
Tropos remained on the Grim Plain, now a god, rather than a Primordial, with ultimate dominion over Undeath and the Undead. Divine, e could not cross the Web of Fate; dead, e could not return to Aiaos; immortal and undead, e could not die. Instead, e waited, feeding on necromancy as a sacred act, teaching those who sought him out to wield necrosis in eir name.
As the god of undeath, Tropos became a focus of the World's necrotic energy, drawing that power in, amplifying it, then returning that increased energy. Likewise, shrines to Tropos serve as foci to gather necrosis, reshaping the world around them to be more like the Grim Plan, with the ultimate aim of readying Aiaos for Tropos' manifestation.
Tropos' first act as god was to attempt to resurrect eir slain fellows - Ramad and Hroun - instead transforming them into undead servitors.