1. I dont play it buy im in a spanish overlord group and the guys that play it there shared this story, so it may not be true but i trust the guy that shared it.
First, These arent the keno and ainz from the side story, they are just based on them, which means these guys have nothing to do with it, so the story goes this way, these stories are what if so they have different routes depending on what you chose, this story goes like this.
The story starts in the Ivernia kingdom (keno´s home), it seems like not all of keno´s kingdom was affected by cure elim yet, keno has lakyus from the adamantite group "Blue rose" as a bodyguard (dont ask me what she is doing here, i dont know lol), they both go in search of an ancient treasure of keno´s kingdom to try and fight "a calamity" that attacked their kingdoms.
Lakyus, keno and a group of magic caster go in search of this item, the find it, it is called \[Galaxy eyes\] it has the ability to see other universes and track something called "hope", they dont know what that is but they believe it is some kind of compass that helps them to track something or someone that could help them, but there is a problem, to activate the item you need to use great quantities of mana to open a portal to connect to the other universe.
2. After some consideration keno puts the item in her right eye, to compensate for the low mana that keno has. the magic casters make a ritual to transfer all their mana into keno, while they are carrying out the ritual "the calamity" arrives, so lakyus push keno into the newly created portal so she can survives.
Kenos wakes up moments later in the middle of a forest, she contemplates that is the other universe is close to her own, then she should go to the adventure guild in E-rantel, she is tricked by some men that promised her to guide her but only wanted to steal from her but keno is a 4th tier magic caste so she defeated them with ease, then by pure luck she meets gargaran and here she finds out that blue rose also exist in this universe and she also learns about the existence of a very powerful adamantite party "Darkness" (if you havent noticed she arrived to the "main" universe, or at least the main universe of the game which it is suposed to be the overlord we all know), after that she notices Lakyus looking for gargaran and she gets sad, she decides to flee and go find Darkness because she doesnt want to put Blue rose in danger.
When she arrived to the adventurers guild she discovers she need to put a quest for darkness to even get to talk to them, but the quest must be of the apropiate level, so she goes to the tob forest and starts thinking about getting a way to orchestrate a quest, while doing this she is attacked by a strange lizard that somehow is completly immune to her magic, she is lucky because these misteryous beast retreats to the forest after a while, so here she gets the idea to get use this monster to attract Darkness.
Now we change perspective to Ainz that is walking around with nabe and hamsuke a few days ago, while being momon, ainz starts feeling weird, he is hearing a strange yet familiar voice in his head, this confuses ainz because he thinks he is being under an attack, which is weird because he should be immune to mental attacks, so he changes places with pandoras actor to see if the voice is an attack directed specifically towards Ainz or momon.
3. Days pass and Pandora notifies Ainz that he hasnt felt anything out of the ordinary nor heard voices in his head in his time as momon, with this information ainz eds up thinking he is either suffering a colateral damage of turning undead or he is going mad, which should be impossible because as an undead he should not be able to go mad.
While in momon room in E-rantel, Ainz thoughts are interrupted by anizach whom has a special request for momon, so ainz changes places with pandora so Ainz can be momon, Ainzach notifies momon that there is a request to exterminate an unusually powerful monster in tob forest, a beast that could be on par with hamsuke, something that ainz brushes off because then it will be an easy job, but a beast like that should be impossible since the forest is under the control of aura and she hasnt informed him about anything like that.
Ainz learns about that the person whom put this quest is a 4th tier magic caster that discovered this beast was immune to magic of the 4th tier, which made this a dangerous monster, this picked ainz attention because if such monster went under aura´s radar it might be worth to capture it and make it a new pet, also ainz is curious about this tier 4 magic caster so he accepts-
This is how ainz meets keno, so she, momon, nabe and hamsuke go to the forest to fight this monster, here is where ainz starts hearing that strange voice again, this makes ainz thinks keno may be the one responsable of this attack so he gets his guard up to kill her if sh tries anything funnny, ainz informs nabe and hamsuke via message to be careful.
They arrive to the forest go deeper into it and they find the creature, but the monster ended up being one of auras pet, [Quadracile](https://overlordmaruyama.fandom.com/wiki/Quadracile), Ainz gets dissapointed because he got his hopes up so they start "fighting", after a while he says to keno that he and nabe are going to use their triumph card, a powerful that will completly obliterate the "beast" in exchange for a lot of their stamina, they do this "special attack" and Quadracile ends up disintigrating withou a trace (they teleport it back to nazarick), which end up with momon and nabe "extremely tired" nabe "faints" and momon is "unable to even lift his swords".
4. Kenos is extremely is extremely surprised in the face of such power, she ends up, while waiting a few hours for momon and nabe to rest properly keno ends up being honest with them, and she tells them everything, she tells them about the magic item, about "the calamity", about her travel, and about her intentions to travel here and find either powerful allies or an item capable of defeating "the calamity".
Ainz, of course, is in complet shock learning about the existence of such item (he even considers kill keno right then and there to take it lol), but he decides against this, momon ask keno if she can describes this calamity, she tells them that it is a person, an evil entity wearing a crimson robe with a hood that hides its face.
The conversation is interrupted by the voice ainz has been hearing, but this time it is clear in his head, it says "I have finally found you", a portal opens, a bunch of undead come from it and start attacking them to try and take keno, they fight but keno gets hurt and faints, she wakes up in momon room, momon decides to help keno defeat this calamity by going to kenos universe and kill him, keno agrees.
Keno thinks ainz is the "hope" her item \[galaxy eyes\] was looking for, which ainz doubts, he ask if his team Darkness existed in her universe, kenos says no, so ainz argues that the function of the item may be that it merely search for the differences between the universes and these variables could be the thing they need to make a difference in the items own universe.
Momon tells keno to prepare for the journey, which keno says that the journey back to her universe may be a little difficult because she needed the mana of at least 10 mithril rank magic caster, Ainz tell her to not worry about the mana and just prepare, while he goes to nazarick and prepares the items he thinks he will need, he also informs albedo about the situation and his plan of going to that dimension and conquer it in the name of nazarick, once he has prepared everything he goes back to keno, which leave albedo confused because she thought she would be going with him wich ainz denies claiming he doesnt want any of the guardians to get hurt, albedo easily folds this time because she imagine as a wife saying goodbye to her husband whom is going to a bussisness trip.
Once momon returns with keno, she reveals she tried to reclute more adventurers but she was surprised to discover all these adventurers were far weaker than the ones in her world.
The plan is to make keno and hamsuke go first so they can find the calamity, once they find it keno will activate the item so momon and nabe can join keno in the appropiate time, this worries kenos because she doesnt have the mana to activate the item even a single time, momon gives her some scrolls that boost her mana by a great deal so she can use the item.
With the plan made up keno open a portal to arrive to her room in the royal palace of her country, but when they arrive keno finds out she is too late, her kingdom is on fire and destroyed all the populations seems to be transformed into undeads and.... the story apparently ends here.
5. There is a second route where momon declines keno request, which means ainz never meets keno and she is alone in the tob forest, well thanks to this when the portal that opened where a bunch of undead came out to try and take keno still happens but this time because keno is alone she is taken.
Months later after this a portal opens in the middle of E-rantel, from this portal keno and "the calamity" come out, momon (whom is pandora in this moment) and nabe are the ones that confron these two new entities, pandoras feels strange towards this individual in a crimson robe, he feels familiar yet different, still pandora notices this individual is very powerful so he orders nabe to flee and inform ainz of the situation, because this new entity is someone they both cant take alone.
Meanwhile this new individual says to himself that this new universe is perfect and it is time to continue his "game of conquest". And with that the second route of the what if also ends.
If you didnt connect the dots this "calamity" is an alternate version of ainz, the funny thing is that this version of ainz is from ANOTHER "what if" from a past event where instead of being an overlord specialized on death magic, this ainz is specialized in time magic.
This is all the information i know of, i dont know if there is going to be more since i dont play the game so im waiting for the guys in the group that play it to tell us if anything new come out.