You can take Cython argument that Mathlann is Manann are the same because they rule the same sea and apply it to the afterlife - Morr and Usirian and the nameless death God of the Ostagoths are all the same guy because there's just the one afterlife. The problem with that is, well, we know there isn't one afterlife. Gazul rules the dwarven afterlife and Ereth Khial rules the elf afterlife (in DL canon, an elf afterlife) and we're pretty sure that those places are distinct from Morr's realm. You could make the argument that afterlifes are segregated by species but are otherwise singular, but it's impossible to know, and it's not even the mainstream belief in the Empire - afaik it's generally thought that each God has its one afterlife, it's just that Morr carries the soul there and also Morr has an afterlife that is the biggest and/or the default.
I think we can all agree that Gazul is definitely His own thing, just like the rest of the dwarf gods are.
But you forget: the elf god that Morr is often compared to is Morai-Heg, not Ereth Khial. Ereth Khial handles the elf underworld, Morai-Heg is the one that weaves prophecy and foresees when people eventually die.
If you assume that Morai-Heg wanted to 'expand' Her domains but didn't want to step on Ereth Khial's toes, then making a human afterlife would make sense because presumably there was no competition - or if there was, She could have stomped or absorbed it. Asides from fate and death, Morai-Heg and Morr both are associated with crows, too. Not to mention the whole story Heidi told Mandred of Ulric wrestling Morr who was disguised as an old woman. That fits, at least superficially.
Usirian is a bit of an odd suspect for being the same individual, at first glance - there's not many direct similarities beyond being an underworld-god. He's associated with flesh-eating beetles, not crows, too, though I suppose that could be excused if the lands of Nehekhara simply didn't have crows.
But there is one giant point of, shall we say,
direct contrast: The priesthoods.
- The Mortuary Cult was focused on prolonging life and defeating death, which led to the Liche Priests eventually binding their souls to their bodies eternally. Because Settra and subsequent kings lusted for eternal rulership, the Mortuary Cult's secular power grew until they became second to the kings. It became a central part of Nehekharan society to the point that the 8e Tomb Kings book describes it as 'obsessed with death', with more and more wealth dedicated to making more elaborate tombs and statues of the dead than used on the living. Finally, the Mortuary Cult forced individuals to join them, particularly the firstborn sons of kings.
- Morr's Cult accepts that everyone will one day die, and they do not seek to defy that. It does not seek out secular power. It is very much decentralized and accepts that Morr is only the god of the dead, not someone who has much to say to the living besides comforting the grieving, and opposing murder and necromancy. Finally, the Cult of Morr is universally served by those who join of their own free will, sometimes guided by actual dreams.
The picture this paints, to me, is that it's pretty plausible that the Cult of Morr was founded or at least heavily influenced by surviving (non-Liche-Priest) cultists of Usirian who did not want to repeat the mistakes of the Mortuary Cult. They kept the central tenet, the preservation and caretaking of the dead, but they went the opposite direction in everything else.
The Cult of Morr's seat of power (such as it is) is in Luccini, in Tilea's coast. It very plausibly could have been the first place where the surviving Usirians landed after fleeing Nehekhara.
The key issue with this idea, of course, is that Morr need not be Usirian for His nascent priesthood to have merely seen/heard what happened with Nehekhara to go '
yikes' and course-correct to adapt to the threat of necromancy - it could just be that they're learning from someone else's mistakes, not their own.
And even if you
do assume these contrasts support Usirian=Morr, they do not actually support Usirian=Morr=Morai-Heg. It's possible that Morr in his current incarnation became the way he was because his priesthood took superficial elements from Morai-Heg and the Ostagoth's death god, without actually being the same person.
Ultimately, none of these speculations can be verified. This is just a theory... A quest theory.
I suppose we could start asking Boney about similarities between Morr and Usirian, but I wouldn't expect any answers to be super enlightening, given that we only have books on the general Nehekharan pantheon, not specifically on Usirian.
Like, Vampirism is a hell of a feat... but the immortality isn't quite as good as Nagash's, and he doesn't care for easier spreading because he recruits in different ways. But he was perfectly able to subvert it.
Vampirism is 'better' than Nagash's immortality if you prefer not being a giant skeleton man, and prefer being able to retain more mortal sensations, and being able to pass for human (in some bloodlines). It's worse in other aspects.
