So, after Dark Messiah, the world is basically doomed. But we still have possibilities to show what happens afterward. This is all assuming that we go with the Hero 5 one race one campaign things.
First we have an intro scenario with the Demons swarming across the land.
Second we have one that is a faction trying to form alliances between all the survivors of the initial strike. To my mind, Haven would be best for this as they now have mostly restored relationships and probably have a big interest in keeping demons at bay. There would be fights against other factions and several timed missions to beat the demon horde to various locals.
3rd, we see another faction, possibly Dwarves, serving as the front line in a delaying tactic/ land grab for fortified locations and chokepoints to try to stall the demon horde long enough for another team, different faction to attempt to gather supplies needed to open a sufficiently large and stable portal to another world/dimension open to get enough of the survivors through to let the remaining races to survive. They successfully claim many of their lost fortresses but a great many have already been corrupted. A dire omen for days to come that such great citadels fell and were corrupted so quickly.
Fourth, Either Academy or Dungeon fight and evade demons and splinter groups/ demon worshipers in an effort to gather artifacts of sufficient power to save Ashan or, failing that, get some of the survivors to a new world to have a chance to start again.
Fifth. Necropolis is fighting a delaying action, but this time we see that things are really collapsing and that time is running low. Most of the defensive strong points are lost and the demons are pushing hard on the ones that remain. It is abundantly clear that only Necropolis has the power to delay the demons for more than a few days. And that is only because they have raised many of the other war casualties. Sandro reappears causing much confusion after his apparent death many years before and does significant damage to the Inferno forces as they were equally unprepared for a necromancer of his caliber to suddenly reappear on the offensive.
Sixth Sylvan or Stronghold claim the rest of the needed artifacts to create the portal but casualties are high, despite their best efforts they just barely get the remaining pieces back to the ritual site.
Seventh, All in.
A coalition of factions, like in the academy campaign of 5, make a series of strikes to attempt to claim a sufficient powerful ritual ground for the portal. No one faction is the star here, and other factions may appear that didn't have a personal campaign story arc. Last mission is holding the area against the unstoppable demon army all involved know this is the last chance, the last gamble, the last throw of the dice.
At the end cut scene we see the portal open and a steady retreat being made. Possibly with certain races or still living major characters realizing that someone has to stay behind to defend the portal long enough for everyone else to escape and maybe then destroy it. With those who remain giving their lives, perhaps their very souls in defense of the portal. The last that is seen of them is a small band of well-known survivors with tides of demons coming at them after the portal closes and then we see the survivors stepping out onto a fresh new world but knowing that it will have challenges and enemies of its own.
Alternate 7 from Demons perspective.
The demon advance is relentless, so far it has had nothing powerful enough to pose a serious threat. But now the remaining forces of the Alliance are gathered at a single point. Kha-Beleth seeks to destroy all remaining mortal races in one fell swoop. But first he must break through the remaining barricades. He must succeed both before the Alliance can complete whatever they’re planning and before another can defeat them and claim the prestige and power that come with it. Ends with a ruthless slaughter of all those who remain. Either with the ritual being disrupted or going off at the last moment creating a worldwide effect similar to what the Heart of Gryphon does, banishing the demons and restoring the barrier between the worlds allowing a few pockets of survivors scattered throughout the rest of the world to rebuild and survive.
The last bit all depends on whether or not we want to keep Ashan. If the answer is yes then the banishment takes place, if the answer is no, everyone is slaughtered and the ritual fails.
Remaining info needed: Dungeon or Academy, Sylvan or Stronghold, Which of the three possible endings, Various characters needed I can probably make some up but I would prefer to use cannon heroes if at all possible.
Notes: Sandro lives because he is a popular character and well known for his powers of deception so it is possible he is still actually alive and uses this deception to allow a rapid strike that staggers all.