Possible reasons for why Mannimarco does what he does in ESO (besides being impatient in waiting for Daggerfall to roll around)
In a world without Mannimarco as the eso villain, he'd just get kicked off Artaeum and be chillin in some cave somewhere for the next 800 years while waiting for Tiber Septim to do his thing. So what happened to make him change his mind about what is by all appearances a solid bet? Especially given that he seems to know his own future to a degree as shown in the book “ Where Were You When the Dragon Broke”. It’s possible he doesn’t recall the nature of Talos until after the Daggerfall break, since the book first appears in Morrowind, but it's more fun this way, so there. And also, this book appears in eso Cyrodiil in a moth priest temple, which may indicate that it was in fact published before Morrowind in-universe. So there too, maybe.
Possible scenarios, in no particular order:
Varen becoming a legit emperor threatens Mannimarco's godhood because if he unites Tamriel without Numidium or Zurin's mantella, Mannimarco doesn’t get a power boost this time around. So he sets up legit emperor Varen as his own personal betrayal of Talos to get his, with Tharn as the observer and the amulet of kings as the Numidium reactor.
Varen is set to become yet another short lived ruler-incumbent in a long line of them, but Tharn is a shrewd and patriotic mofo who unwittingly threatens to destroy Tamriel in his bid to force Akatosh to step up in picking a new favorite [via his original intent as eso's big bad], so Mannimarco backs Varen in an attempt to brute-force his own betrayal of Talos, with saving the world as a nice bonus.
Vanus is stepping up his persecution of necromancy and Mannimarco is feeling the pressure, so in an act of desperation he pushes his timetable for godhood forward by a few centuries in an attempt to solve the problem once and for all. I.e. he goes in early so neither Vanus nor any other earthly power can take necromancy away from him or his followers. Btw, that’s why [I headcanon] he wants to become a god even in Daggerfall. Necromancy in all its forms, sordid and otherwise, is his Thing, so he naturally wants it to live forever and he feels he is the right mer for the job. He's like Van Gogh, but with dead shit. (Or maybe more like the Marquis De Sade...)
Mannimarco doesn’t actually know how he becomes a god, only that Zurin Arctus and some bitch named Hijalti have something to do with it. He’s finding his way to the sky ass-first, and trying to swallow Bal is just his latest hare-brained attempt.
While visiting Orsinium on a necro-promo tour, Mannimarco discovers the titan and is floored by it. He has his cult raise the titan and manages to summon the sleeping spirit of the dragon itself for a chat. It rages at him about it's mistreatment and vows that their Father would eat Bal alive for this grave insult, which gives Mannimarco an idea. Vanus's increasingly hostile and systematic oppression of Mannimarco’s favorite pastime may or may not encourage the early and hasty implementation of this idea..
So, why does Mannimarco need Varen, et al specifically? He wants to find that rock and use it to pseudo-mantle Bal, but why does he need new emperor Varen and his three highly trained and loyal private bodyguards to do what he did? He could have conned literally anyone safer into insulting Akatosh enough/doing that mysterious ritual to open the gem. Tharn even does it with us. He says himself it’s the same ritual. So why those four?
Talk about high risk for little reward. He didn’t even need them to find the stupid thing. None of them knew where it was or had any kind of intel on it or else they wouldn’t have had to go on a quest to find it in the first place.
(You would think it would be the Blades' job to secure it after each emperors death and courier it to the location of a potential chosen one for inauguration purposes, but no, apparently not. Wtf are you guys for then???)
Given the above, the reenaction of the betrayal of Talos is the least stupid option I think, even if it doesn’t quite make sense that we can do the same with our five companions. Unless the entanomorph doesn’t need the "betrayal" to be a secret? Of course, we don’t become a god (that we know of any way lol), so maybe the ritual we do isn't exactly the same, or Mannimarco just really really likes to make his life harder than it needs to be.
