(Excuse any lack of deep lore knowledge; its multifaceted and dichotomous nature sometimes spills from my brain and obvious things get forgotten)

This is a condensed blurb of my thoughts so please feel free to expand and correct me.

It is my impression that the return of the Numidium is what is called Landfall. The Numidium then proceeds to keep annihilating the Altmer and they keep fighting it. The Numidium employes it ancestroscythe and the Altmer have no way to fight it since it essentially (ironically) deletes them (by killing their ancestors (their gods? their progenitors? up to which gradient/level???).

The Altmer then flee to the moons if I remember correctly but the Numidium eventually reaches them there too.

So are the Altmer/Thalmor just protecting themselves? This all started by Talos/Tiber activating the Numidium. He started it all with an act of aggression.The Thalmor are just defending themselves. The best defense is taking out the enemy/ they are trying to delete Talos, prevent the Numidium from sieging Alinor for millenia and defend themselves from being deleted.

But then again the Altmer wanted to get back to Dawn/unmake reality from before the third era anyway? Or could this be explained by the weird time bending realities of dragon breaks etc.

The Altmer would feel that men have seriously messed up by activating the Numidium (ah the younger races acting with such rash passion and emotion), and in essence is it Men who are destroying the world by activating something beyond their understanding. The Aldmeri Dominion would want to step in and correct the mistakes (Or so they would thing). Hence go in, do some cleaning, deleting, ensuring no Talos is formed, no Numidium and while you are at it clean up men so they don't make the same mistakes again. The Elves win and save the world (some might say here that they don't want to save the world but annihilate it. Here perhaps I would say that they want to get back to only a few subgradients after creation so still a part of the world, just closer to the dreamer. Men's (Talos and Numidium) actions would potentially wake up the Dreamer and unmake the world?)

Sorry for the rumbling and possibly disconnected ideas. This theory kinda ignores some of the other things that take place, I feel, but please feel free to discuss it and help me along in my thought processes.

Cheerio