I've been away from the forums far too long...anyway, I feel a need to post this mod idea and so get it out of my mind and share it with the world. I am in no way a Dark Souls scholar or pro-gamer on these games who has explored every last inch and tried every single combination possible to play. Just an enthusiastic player of Dark Souls and Mount&Blade.
Dark Souls for Bannerlord (I haven't played Elden Ring yet but presumably one could make a mod out of that one as well)
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Dark Souls to Bannerlord
Presumably one could set the game during, almost any point in the timeline but here I'll assume its placed during an unspecificed point between start of the first and conclusion of the third game. The main difference is that the Chosen Undead/Ashen One is not a loner but a leader who leads an army to, well, do her thing. And same with her enemies. They are also bringing a bunch of their friends to the fight.
Main features making this mod different from other bannerlord mods
A dying world
Perhaps one of the greatest differences would be that the world in this mod would not be a thriving one with trade caravanns, village life and so on going on but a world dying and the light is fading into darkness. Now this will not be implemented to an extreme which would risk leaving the player character to find no one to fight as everyone has been killed off in AI-vs-AI fights. However it would mean that the "economy" and recruitment would work in a different way. Mostly notably in that Souls gained from battles and quests, not "money", would be used as currency and for other purposes but more on that further down.
A greater world of Dark Souls
Only a few places in the Dark Souls world are visited in the game and a few more are mentioned. Hence beyond these parts the developers would probably have a pretty wide freedom to add lore friendly additions to the game to fill out the blanks and so on. This might be a boon or a drawback, depending on personal preference.
The Undead
The player would, naturally, be an undead and according to lore essentially impossible to permanently kill off. This might be a bad thing for some and so offer an option to follow the TT Dark Souls rpg in that every time that the character gets killed there's a chance he'll go permanently off. If one would do such then its curtains as the personality, consciousness and thinking would not survive and the player would be reduced to a murderous undead who attacks any and everything close which is "alive" and isn't also hollow. Failing to have anything to attack will mean the hollow just kind of stands around and waits for something to show up. It will exist as such to the end of time, and possibly beyond that.
Ideally one could do something with Humanity as well but that could be beyond the engine so one might need to skip that. Or one could use the humanity one has gathered in order to make troops go from weak hollows to more powerful, and human looking, undead versions of themselves. But again, I don't think the engine could facilitate this part.
Undead troops, there would be no living humans around, would never, or seldom, be permanently destroyed but either captured or reduced to hollows and then either recruited or taken, depending on the faction, to permanent capture in Undead Asylums and other horrific places where the undead are to wait for the end of the world.
One thing one could run with is that also undead troops are limited in number. Still numbering in the thousands but not spawning new ones and so it would make troop management very different as sooner or later new undead soldiers would only be found as recruited from prisoners or from capturing enemy strongholds. And other forms of troops won't be growing on trees either given the state of the world. I'm open to if this should be included or not as it could be lore friendly but also very annoying to in late game discover that the game can't be continued due to no troops being recruitable. Still it would make the mod very different from other Bannerlord mods and campaigns.
Bonfires
Whenever a character is killed, they return to a bonfire rather than risk getting captured and returns with all their gear intact. A pretty forgiving part but can be made more hazardous as suggested above.
The Abyss
The threat from the Abyss would exist and if it overtakes, or say a Darkwraiths faction captures, a settlement then that places becomes off-limited to everyone else as the Abyss messes with it beyond saving. One could fight or embrace the Abyss but there likely won't be a neutral option in regards to it.
Souls for Profit
I would think that Souls replace money and that souls are gained from fighting and then used to get stuff from merchants, upgrade troops and ideally to upgrade the character although I would think that the game engine would prevent this. But also that killing special enemies empowered by specially strong souls would give the player special weapons to be equipped or given to companions to create killing machines with iconic weapons.
But one cool option could also be that the player can imbue a soul like this and "force" the player to wear a special armour set which grants some significent boons in addition to looking the part. If you imbue Ornstein's soul for instance you get the look of the Old Dragonslayer Armour along with nice boons to your stats. It would get even more wild if you got your hands on a Lord Soul.
Sorcery, Pyromancy and Miracles
Magical would naturally be present in the three main forms across the three games. I left out hexes as that only, to my knowledge shows up in Dark Souls 2, but I understand that other mods have found ways to include magic in the game so this shouldn't be a big problem.
Dragons, demons and more
There are many non-humanoid creatures so this might force some work but to my knowledge it should be doable. Even if I could be wrong here.
Many creatures are fougth 1-vs-1 in Dark Souls but in this mod they would naturally brings friends to the fight. Thus facing down, for example, Ornstein and Smough would mean fighting them and their army of Silver Knights in or outside of Anor Londo. Same with seeking out the Gravelord for the Death Soul as that would mean an army of skeleton warriors, skeleton creatures and necromancers and so on in addition to Nito himself.
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I could write and write on this topic but I want the post to be readable and will thus stop here.