Morr and Verena play a significant role in LUMW, arguably greater than that of Taal and Manann (and certainly greater than Rhya's). This is notable since LUMW is an Ulrican tale and so might be expected to give greater significance to Ulric's pantheon than to the Classical Gods, leading me to suspect that this is a remnant of the earlier legends that contributed to LUMW rather than a later alteration by the Ulrican editors. What really draws my attention to Morr in particular is this one sentence describing the aftermath of the battle against Chaos:
But the victory was not without cost. Thousands of the gods' servants lay dead. Lesser gods and heralds had been lost forever from the world. The Dragons that had fought with the gods had seen most of their number fall. And worst of all, King Morr himself was gravely wounded. He lived, but was forced to find succour in the Dark Lands and was never more seen on this earth.
This is an utterly bizzare sentence. What do the Dark Lands have to do with Morr? This doesn't seem to make much sense as an account of actual events, but this also doesn't make sense as an alteration by Ulrican editors, because what purpose does it serve? As far as I know there isn't any modern orthodoxy - Ulrican or otherwise - tying Morr to a specific geographic location, least of all the wastelands east of the Empire. And why is Morr singled out for not being seen on the earth, as if the other Gods are still hanging around? Morr dying (if that's what this is) could maybe make sense since He is the God of death, and there is a myth about Him being murdered by Khaine apparently, but again what's with the Dark Lands?
I think that this particular phrase is likely to be a mistranslation or misunderstanding of whatever sources LUMW was collated from, and I propose two possible explanations for what might have been originally intended.
Morr is a dragon
The sentence about Morr finding succor in the Dark Lands appears immediately after the sentence saying that most dragons that fought with the Gods died. The Dark Lands don't have much of anything to do with Morr, but they do have something to do with the dragons, as the Dark Lands contain the
Plain of Bones, the ancient graveyard where dragons came to die. According to Deathfang and other sources some dragons treated with the Gods more or less as equals, but that certainly isn't clear from LUMW which barely mentions the dragons. Perhaps the original sources of LUMW told of a particularly important dragon that died, or went to the
Plain of Bones and was presumed dead, and then the editors edited that to be about Morr because the idea that the fate of a dragon is as important as the fate of the Gods didn't make sense to them. None of this is to say that the God Morr currently worshipped in the Old World is a dragon, only that it's possible that the figure called Morr in LUMW - or at least in this particular sentence in LUMW - is in fact not Morr but some dragon.
As for which dragon this might have been, I propose Kalgalanos. Deathfang told us that Kalgalanos was one of the five flight leaders of the dragons, and that after the battle of Chaos he was "never seen again", which resembles the line about Morr "never more seen on this earth", and as one of the flight leaders of the dragons it makes sense for Kalgalanos to be important enough to be mentioned. Furthermore, Kalgalanos was known as 'Kalgalanos the Black' and Morr is associated with darkness in LUMW, and in OS is called 'Dark Morr', so It's possible that the epithet of 'black' lead to his conflation with Morr.
Morr is Morai-Heg
At the very start of LUMW the listing of the Gods seems to put Taal and Morr as parts of different hierarchies. Taal and Rhya are described as "Father Taal and Mother Rhya", while Morr is described as "king of the darkness" and Verena as "queen of the light". Later, when Ulric asked Ranald to call for help, it is specifically noted that he should inform Taal and Morr:
He called to his brother Ranald to immediately run to tell Father Taal and King Morr of what they had seen, so they might make ready their armies to drive back this horde.
This lines up with the idea that there are two pantheons, one lead by Taal and one by Morr. From a modern perspective it seems like an obvious allusion to the seperate Classical and Old pantheons, but I suspect there may be a deeper truth hidden there. There's other reasons to believe that there are at least two different pantheons of Gods: Deathfang's mention of Rhya and Isha being seperate and belonging to different groups of Gods, as well as Mathilde's investigation into Karnos indicating that Taal and Kurnous are seperate, and finally the recent side story. All of those seem to imply that there is at least one seperate pantheon from the elven pantheon.
Assuming that Taal and His family are that 'other pantheon', it follows that Verena and Morr might be elf Gods. If Morr is an elf God it's not exactly a mystery which elf God He may be; Morai-Heg, Goddess of death and fate, has a similar domain and a similar name to Morr, and it's a not uncommon in-universe theory that they are one and the same. She is also of the Cytharai*, the elven Gods of the underworld. Verena is explicitly said to be Hoeth by MF, and Hoeth is of the Cadai, the elven Gods of the heavens. This could give meaning to the phrases "queen of the light" and "king of the darkness": during the time of the Coming of Chaos Morai-Heg was leader of the Cytharai and ruler of the Underworld while Hoeth was leader of the Cadai and King of the Heavens.
There's arguably some evidence for that. As far as I know there isn't a canonical "leader of the Cytharai", but there probably is a ruler of the Cadai: Asuryan, Emperor of the Heavens. Asuryan isn't Hoeth, of course, but in both OS and MF Asuryan is thought dead for most of the fight against Chaos and only comes back at the very end. In the Asur
pantheonic Mandala the runes of the Cadai are all topped by crowns (except Ladrielle), with Asuryan
and Hoeth also having five points above Their crowns. Perhaps this is a remnant of the time Hoeth served as interim leader of the Cadai in Asuryan's absence?
How does this explain the matter of the Dark Lands? If Morai-Heg truly was the leader of the Cytharai, perhaps some source used the phrase "king/queen of the Black Pit/Underworld", the Black Pit being the common translation of 'The Mirai', the name of the elven underworld. Something might have been lost in translation as Black became Dark and Pit became Place or land or something of the sort, and an Imperial editor might have grasped for something familiar and found only the Dark Lands. Morr never being seen again upon this earth isn't entirely explained by this, but I suppose this could be a poetic way of saying that with the Vortex completed and the world drained from magic Morai-Heg could no longer have an avatar in the world (though that begs the question of why Morai-Heg in particular is singled out).
Some other stuff
Was Morr present in the final stand?
OS begins by saying that King Taal was advised(?) by Morr:
And then the Cataclysm came. King Taal rose from His Forest, and with Dark Morr muttering dire portents in His ear, He banished all immortals from the world.
Later on the Gods taking the final stand against Chaos are listed, and Morr isn't among them. This seems to be in contrast to LUMW, which says that Morr did fight against Chaos. Perhaps LUMW is right about Morr not being a great warrior, so He just provided some intel but didn't take part in the actual fight. Perhaps LUMW is right about Morr being heavily injured, so while He was there for the start of the battle He didn't last until the end of the fight.
King Taal and Asuryan
MF says that Taal inherited the rule of the world after His father Asuryan died. This makes no sense if we assume that Taal is actually Kurnous (remember that MF is an Asur tale), and it makes even less sense if we assume that Taal is Taal. One possible interpretation, if we accept the two pantheon theory, is that with the leader of the elf pantheon dead the leader of the non-elf pantheon assumed leadership over all Gods. The side story also seems to be suggest that the elf pantheon might have been in better shape during the Coming of Chaos than the 'regular' pantheon, which might explain why Asuryan was the leader of the Gods before His apparent death.
Khaine and Morr
There are somes myths about Khaine being Morr's brother, and/or of Khaine killing Morr. I think it's not hard to argue that Khaine is the most prominent Cytharai. Perhaps Morr is Morai-Heg and was leader of the Cytharai during the Coming of Chaos but lost Her position to Khaine after it, which could perhaps explain some of those myths.
*Some later canon claims Morai-Heg is neither Cadai nor Cytharai, but Boney seems to discount that as being pretty dumb, and has listed Morai-Heg as Cytharai.