Posts from RTF is a global modification that replaces all towns in the game.

Is Linna's death mandatory?! - Where did that come from? If new campaigns try to stay within the lore, their plot should not exclude all possible endings of Dark Messiah!!!
- Can the player ensure that Linna survives? Answer: Yes, they can!!!
- Can the player ensure that Linna dies? Answer: Yes, they can!!!
Therefore, the human race can fall, or it may not, but it is not specified when exactly (tomorrow or in a hundred years), meaning that humans must exist, but it is quite possible that not in the form of a unicorn empire, but as part of something else; moreover, the fall of the human race does not imply extinction as a species, but implies a fall as an independent empire.

I found this information a long time ago in the comments on the website for the 7th Heroes game:

Sareth did free the demons, but they did not conquer the world; instead, they became, let's say, "aggressive neighbors" to the other races. With the arrival of the demons in Ashan, the cycles of eclipses ended, and cycles of the Void began. In general, the demons became Ashan's enemy number 2, and the Order of the Void of Sandro took the dominant position. I have no idea what the future holds for this. The information was taken from Erwan's presentation on Ashan.

Is Linna's death mandatory?! - Where did that come from? If new campaigns try to stay within the lore, their plot should not exclude all possible endings of Dark Messiah!!!
- Can the player ensure that Linna survives? Answer: Yes, they can!!!
- Can the player ensure that Linna dies? Answer: Yes, they can!!!
Therefore, the human faction can fall, or it may not, but it is not specified when exactly (tomorrow or in a hundred years). That is, humans must exist, but it is quite possible that not in the form of a unicorn empire, but as part of something else. Moreover, the fall of the human faction does not imply extinction as a species, but implies a fall as an independent empire.

Level 1 - dwarves (Guards/Legionnaires/Sentinels), levels 2, 3, 4 - humans (Archers/Veteran Archers/Light Archers, Warriors/Crusaders/Clerics, Novices/Faith Protectors/Vestals), level 6 - elves (they will be unicorn riders).

I found this information a long time ago in the comments on the Heroes 7 website.
Quote:
Sareth did free the demons, but they didn't conquer the world; instead, they became, let's say, "aggressive neighbors" to the other races. With the arrival of the demons in Ashan, the cycles of eclipses ended, and the cycles of the Void began. In general, the demons became Ashan's enemy number 2, and the Order of the Void, Sandro, rose to a dominant position. I have no idea what the future holds for this. The information comes from Erwan's presentation on Ashan.

Official source or a recording of that presentation, please!!!

Official source or a recording of that presentation, please!!!

And in general, in this case, it is necessary to leave the demons in the game, and such an ending is clearly nonsense! - the demons are finally freed, the most patient of the inhabitants of Ashan rejoices! (I'm talking about the Overlord) and finally they can... just sit in Sheogh and forget about Ashan... - what, excuse me?! and the Order of Void appeared again out of nowhere - it seems to me that the person who left that comment misunderstood something or came up with it himself (or it's the "broken telephone" effect) to make such statements and rely on them, concrete, verified data from an official source are needed.

I see it this way: it's possible that Sareth will usurp the throne :) but he probably doesn't want to destroy everything - now he is a king and will rule his people.
And regarding the Order of the Void:
1) Lucretia is still alive.
2) In Heroes 6, Sandro found the Ring of Sin, moved to its dimension, retrieved the Mask of Deception, and the Ring's world was destroyed because the soul that created this world went to Asche after losing the mask. In Heroes 5, the Ring's world still exists, which means a new soul is holding its walls. Considering Sandro's phrase, "the ring is now useless, but I will continue to study it," I concluded that Sandro's spirit now lives in the ring and is waiting to be released.

My colleague and I (fktifzobr@mail.ru) discussed the Factions and agreed that we should make Haven not a purely human race, but a mix of dwarves, elves, and humans.

Added 2 minutes ago
Here, I found something:

The Overlord is vulnerable only to Railag (remember the Heroes 6 campaign), so Sareth probably won't be able to overthrow the Overlord (he clearly doesn't have enough strength), so we can't say anything specific about the demons. If Sareth freed his father, then he could have done so on certain conditions, but it's unlikely that the Overlord would listen to him (well, this is just my opinion, but it seems very obvious to me).
But the guess about the Necromancers is interesting - how can the Ring have its own inner dimension, as in Heroes 5, if it was destroyed in Heroes 6? I think there's some truth in what you're saying.
It will be difficult to keep everything within the lore, and if we believe that unverified information, then we currently have three sides: the Order of Void, the demons, and everyone else. By the way, frankly, I find it hard to believe that we can combine dwarves and darkness within the lore - dwarves would rather go underwater to Shalassa than hang out with the Faceless Ones.

Sareth did free the demons, but they didn't conquer the world; they became, let's say, "aggressive neighbors" to the other factions.

I believe that such an outcome is only possible without a Lord. Let's say Sareth and Zana decided to lock up the Lord, but there are many ways to summon demons:
1) opening a gate
2) summoning like Fenrig Zana
3) runic gates in Grimheim
4) a permanent portal, like in the Arantir Campaigns (where a demon would appear once a week)
5) the Griffin's Heart
And it's unlikely that the developers in the sixth installment would just reveal Zana's image for no reason, showing that she is relatively good.

reasoning about demons is, of course, good, but then they need to be introduced as a separate race or...
or someone turned to them for help (it turns out to be Sareth from the Alliance of Light, he might have needed the help of demons (however silly it sounds) against the Order of Void (a campaign for which is simply necessary))

Added 1 minute ago
and then, through scripts and summoning neutrals, a campaign for Sareth can be made, but how does this relate to the magical blade and Lethos?

Let's do it this way: After killing Arantir, Saref teleports somewhere to create an army of demons, and we'll leave the old storylines as they are, but without him. After the Unicorn Empire is destroyed and the alliance of three races (humans, elves, dwarves) is formed, he will appear in Grimheim near the Shio gates, and there will be a super battle, after which all existing Factions will be seriously weakened and won't have time to deal with the surviving demons. Saref and his followers will take over the Dungeons under the ruins of Talongard and establish their capital there.

Before the super battle, the balance of power will be as follows:
1) The alliance of Letos and the Ice King (Chimera mages, Dark Gnomes, Ice Nagas of Shannara, neutral mercenaries from the free cities, and the Black Guard, who switched to Stonehelm's side after Arantir's death)
2) The remnants of the old world (elves, humans, dwarves in a single castle, elementalists, possibly nagas)
3) The Voidlings
4) Proud loners
- Stronghold (with the arrival of the Gnolls, the fragmented Clans will unite under a strong leader and be able to compete in strength with any race)
- Orcs (the Ice King's army will unite into an alliance only after the orcs are destroyed (a common enemy will bring anyone together), after which the orcs will hide in the Dragon's Tooth Mountains, and the Desert Orcs of Sahaar, on the contrary, will create a powerful state)

As a result, the demons will cease to be a serious threat for about 200 years and will give way to our races.
It turned out well:D

in principle, it makes sense, but I think that for this, demons should be introduced as a separate city (which is not allowed by the first rule of the topic) (but their city would be more logical, especially since there are a couple of ideas)
if we proceed from your suggestions, then it can be done so that after Saref took the skull, he was teleported, and the demons remained in Shio, but the Dungeon began to weaken (just about 200 years later and it will collapse)
how about this option?

the outcome is already predetermined! if Dungeon starts to weaken, then even after 200 years, the Overlord will break free and destroy everything, and that outcome doesn't suit us. I still think it would be better to make demonic portals as treasure vaults and external recruitment buildings, let them be locked in Sheoh, and Sareth, as a creature connecting both of these worlds, will summon them.

Frankly, based on the cutscene we see when we decide to free the Overlord, it can be said that the son welcomed his father with open arms. He is unlikely to set any conditions for his release, let alone overthrow him and usurp power. So, we can initially show K'ha stepping onto the land of Ashan with an entourage of several legions of demons, but then something happens, and the demons are forced to postpone their attack on Ashan. I think that "something" could be the appearance of the Order of Void, which solves the problem of the pesky demons in a well-known way. The demons retreat to Sheo, and in addition to seeking revenge on all of Ashan, their goal will also be to take revenge on the Order of Void (we have three opposing sides: demons, voidwalkers, and the alliance). This way, we won't break the lore too much, and we'll create the necessary conditions.

in general, everything depends on the player's choice; in a map for Heroes 5, Dark Messiah has 2 endings, only 1 of which is suitable, and in the game Dark Messiah, there are 4 endings, and 2 of them are suitable. I believe that the "good" ending with Zana is the most logical.
1) Sareth himself says that he enjoys transforming into a demon - personally, I see this as him accepting his true nature and is unlikely to repent.
2) judging by Sareth's reaction to the entry in Arantir's diary, he is against the mass murder of innocent people, which means he will not release his father.