I'm guessing that most people would find answer on this question quite obvious - it was a child of Demosthene and his slained wife (who "fell in love" in one of demonic lords in Disciples 1), possesed by Bethrezen's spirit. It's very strongly suggested especially in Undead campaign, since Uther's blood was used to ressurect Gallean (it's quite obvious that such act would require a "catalyst" of great power, blood of being possesed by Bethrezen seems to be powerful enough to perform ressurection of Gallean.

HOWEVER - campaign of Legions of the Damned is clearly suggesting, that Uther was just an impostor and wasn't "incarnation" of Bethrezen, since he "tricked" Legions and at the end we need to kill Uther in this campaign as well.

I feel a bit confused at this point. Almost all campaigns are suggesting that Uther was "carrying" Bethrezen inside, but not Legions campaign. Add-ons are rather focused on implications of Gallean's ressurection, so this topic wasn't developed further, if I didn't miss anything.

Any ideas?

This is just my understanding, so I may be wrong here. From what I understand, the ritual to transfer Bethrezen's soul into Uther's body was corrupted (interrupted by the dwarves if I recall correctly). So Uther is still "himself", corrupt by the ritual which transferred Bethrezen's strength but not his actual soul.

Everything, abs. everything explained in D1 and D2. No Bethrezen himself in Disciples 2, no Gallean in D2 GR, Solonielle not a same as Mortis.
<<fell in love>> -> possessed (!!!111). It is Demons, almost all of them do not have "sexuality" (both incubus and succubus genderless creatures).
Undead failed Act 2 for Uther blood. Later he give it to undead himself, probably in secret from Demons. Later some Demons allied with Undead (ofc some still loyal to Bethrezen, if you play all campaign simultaneously you know differences between "demons and demons").

Uther probably not a son for Demosthene ... :facepalm:
P.S. Also some russian player wrong think that Drega Zul mother in love with Nosferat (named ...?). Arhh, where they all find this... Drega Zul -- unborned undead child from killed (undead) mother.

" no Gallean in D2 GR" What ? Whatever you wanted to say here, Gallean did appear later on , and it's literally called Gallean's return !
And yes, Sollonielle is MORTIS ! The lore says that, the undead say that, (the elves are not aware of that, only the undead mostly know of this), MORTIS says that, she is literally Gallean's ex wife !
Demons don't have sexuality ? I remember seeing 0 sources for this, demons need to reproduce and they do have sexuality (incubbus is male and succubus is female) and the description of the succubus does say that she still clings to the pleasures of mortals.And since it's a freaking succubus we know what kid of pleasures those are, they are not talking about whine, so why say they are genderless when their freaking forms are form male and female ?
We don't now how much Drega Zul's mother loved the nosferat who made her mother a vampire, her name is Zezebel, but we know that the nosferat who did this was really enchanted by her and liked Zezebel, but had to resort to liking Drega Zul since she was the surviving half-vampire(or dhampir) daughter of her vampiric mother who died due to the fact that her daughter resisted the vampiric conversion while in the womb.

Yeah, Bethrezen couldn't break out of the prison, he is still there in disciples 3 ! There was even the case in disciples 2 when the legions invaded the elven lands in search for Soloniele's tear(a mana well) which would be used to create a parmanent link between hell and Nevendaar, and even then Bethrezen was not free, all if permitted him is to expand his power into Nevendaar ! Thus releasing an infernal plague upon Nevendaar, systematically exterminating all life, leaving
the Legions to rule. The land of the living would become a full reflection of the horror of Hell.
So yeah, the point of the ritual was for Bethrezen to use Uther's body as a host, since his actual body/being was trapped in hell, and this was not a easy thing for such an entity, so transferring his soul into another being was a fix for this situation(similar to how diablo does usually) ! But, for some reason, the only thing that happened is that they only transfered part of Bethrezen's power into Uther, and not his soul, leaving Uther with a lot of power by trying to usurp Bethrezen. I can only imagine Bethrezens reaction : " Okay..sooo..what happened ?/ Demon : Well, sir, we did do the ritual, but the only thing that happened is that the boy(Uther) got part of your power, and you are still stuck in this prison !/ Bethrezen : DAMMIT !
So, as the campaign explains it, Uther was basically leeching Bethrezen's power like a parasite does, basically killing him slowly(why kill him ?)- maybe it's cuz that hellish prison is a damn burden for him, and he needs that power to survive !
But why would a mortal be able to do this to such a powerful entity, like Bethrezen ? Well Uther was most likely able to do it due to the nature of that ritual that had to get Bethrezen out in the first place, which was a ONE WAY power and soul transfer towards Uther(the future host), but it's mostly broken and the only thing that happened is that it constantly transfers power to Uther, leaving Bethrezen dry. This is basically much worse than someone stealing your electricity !
Basically, no matter what was intended, this is what happened ! Bethrezen wanted to escape the prison by possesing another body, but the only thing that happened is that his power was constantly being transferred to Uther, while Bethrezen himself still remained in that prison, leaving him to die there powerless.