Challenge: (Ben10 x Overlord)
What if the Omnitrix got send to another dimension specifically into the New World around the time Nazarick arrived at the new world, and by chance during the time of Momonga and Demiurge flight stroll at the night sky, Momonga saw the spaceship falling to earth, and took interest in it. At that time the Omnitrix immediately attaches itself to Momonga's body...
However, abnormality begin...
[Unknown force from the host confirmed! The Force makes it difficult to attach with the host!]
Due to unknown properties of Momonga body who is tied to Yggdrasil rules and preventing him from using equipment not in his class, Omnitrix received unknown resistance to attach itself to Momonga...
[Proceed to transform host to eliminate resistance, transform the user into a form that can wear the Omnitrix, user current form and DNA will be uploaded to the Omnitrix. Searching out of the database to find a form that will suit the host, the Host confirmed a Magic using creature, list of species with similar abilities. Geochelone Aerio, Thep Khufan, Human (Ledgerdomain/Earth Native). Out of the sample, the form that will cause less disturbance to the host is human.]
Thus Momonga transformed into a human, albeit a magic-using one from Ben 10.
Momonga can access his original form but it will only be a few minutes, and his human form is weak by Nazarick standard. Mind you he will still use magic and stuff so he is pretty strong, added to him going to be peak human physique and mana manipulation. But without racial traits so he loses some amount of his status and skill. The thing he got hit with the most is his mental filter which causes him to be in more panic around his guardian. In return, though Momonga no longer tied to Yggdrasil's old rules in which he can't learn new stuff because he has no class in it or he can't hold weapons, not in his class. It will be useful for Ainz to familiarize with the world from the point of view of a human(Albeit very strong and protected by Guardian 24/7).
As for Momonga original form, as well as all alien forms in Omnitrix Momonga will have access to some of it, but he will have an energy limit or time limit just like Ben in his early days. As time goes, Momonga will understand more about Omnitrix he would unlock all DNA inside it and gain access to his old form along with all aliens inside, but in the early part, he will be human and his guardian will be overprotective of their lord despite him still awfully powerful by New World standard.