It's essentially a mimic of what happened to Akatosh and Lorkhan, and in turn Anu and Padhome. And mimicry of mythic sh*t can have grand effects in the world of TES. Here is my take on it:

Someone, usually a king of some sort, will be secretly murdered (which has a double meaning) by a rebellious upstart. Perhaps the most famous of these is that of Talos. Hjalti Early-Beard, who is the TRUE Tiber Septim in a way, and Zurin Arctus betray Wulfharth, who is the current Underking, by leading him into an ambush; for the record, this is the "color of betrayal" mentioned in the Five Songs and the Arcturian Heresy because, according to said Songs, Voryn Dagoth lead the Shor-led Nordic armies into an ambush under Red Mountain. That ambush results in Wulfharth having his soul trapped into the Mantella, a soulgem used to power Numidium. This event also has Zurin killed because before he turns to ash, Wulfharth's heart roars a hole into Zurin's chest where HIS heart is. In my eyes, this makes Zurin become the Underking in Daggerfall.

Just in case it hasn't clicked yet, the Underking is an avatar of Lorkhan.

In TES II, Zurin the Underking is running around saying "the Mantella's my heart blah blah blah" because it technically IS, so when he reunites with it, Talos is created. Why? Because the aforementioned events mimic what happened to Lorkhan: heart removal and betrayal of oneself to themselves (at one point, people think Tiber is in two places at once because of the placement of the Underking and his armies. Recall that Akatosh is Lorkhan, as well). Tiber (Akatosh) has Wulfharth's (Lorkhan) soul ripped out and placed into the Mantella (Heart). So when the Underking (Lorkhan) gets the Mantella (Heart) back, it brings crap full circle.

Someone will probably come up and chip at my response and clear some crap up for you that I either screwed up or missed.

edit: The "secret murder" I mentioned not only refers to assassination, but by "joining" with a person in such a way that you become One. You join in such a way that no one can tell the difference between the two of you. Examples are the confusion caused at Cuhlecain's assasination (some claim Tiber himself died) and how, at one point, people thought Tiber was at two different places at once courtesy of Wulfharth. The "secret murder" crap also appears in the Sermons where Vivec talks about marriage, and after said mentioning of marriage, he says he means secretly having something murdered.

I don't unterstand none of your posts way to hard for a simple kid of 15 years old

anyway proweler pls don't make names shorter... makes it even harder to understand xD

and how long have you been in the TES universe... like seriously.... as in... knowing about lore and stuff....

-I don't really get all those ashes things... so someone is killed in battle and then re-lives?

-In the books they often talk about Shor and Lorkhan (they're the same god right, the one who convinced the nine of making Mundus? is he also the Allmaker of the Skaal) that he just walks... like he had a body, while I always thought about the gods of being a spirit-like thing...

-Was this storm that protected Tiber Septim during this battle the Underking... (And Ysmir, the guy who leaded the atmorans to Tamriel, is the Underking right.)

-I still don't really understand when someone has a body or is more like a divine spirit or something else... at one moment i think the Underking is a storm the other moment just a person...)

I didn't pick a story based on personal preferences: I picked my idea based on lore, and lore suggests Tiber Septim is more than one person. This is why I said he is and isn't from Atmora. the Underking (Ysmir) is. Hjalti isn't. Both of these people are Tiber Septim. It appears you have misinterpreted my theory.

Atmora was a continent north of Tamriel. Its people started immigrating south to Tamriel during the Merethic Era, and continued doing so until early in the First Age when their home continent was rendered uninhabitable during the First Age due to severe climate change. The people of Atmora who came to Tamriel eventually became known as the Nords.

As for if Tiber Septim came from Atmora, well, maybe. He also could be from High Rock, Skyrim, or the Imperial Province itself. The truth is that there are multiple versions his origins and we have no definitive proof that any one is more valid then the rest. What that means is that we can pick and choose which one is accurate based on our own personal perceptions and prejudices about Talos's character, like how 946000 has chosen to believe the Wulfarath version , or how I've chosen to believe that he was merely an Imperial who became a General for Cuhlecain.

Also, if you feel like being really trippy, you can choose to believe that they are all true because the Dragon Break that occurred during the battle of the Underking against the Numidium retroactively allowed multiple different histories to be correct.

I think I'm getting to understad it.... as far as being able to seperate Ysmir - Tiber and Arctus..... Though at the moment I'm still confused what Ysmir has to do with the fact that Tiber became Talos....

There is just a lot of name-confusion going on here.

Hjalti Early-Beard: Young general from Alcaire who served in Cuhlecain's army.
Ysmir: Ancient god-hero of Men, who is reformed time and again and is associated with the raw powers of storm and shouts.
Talos: 'Storm Crown', a title bestowed upon Hjalti by the Nords after they watched him display his powers at the Battle of Sancre Tor. It is of course no coincidence this happens shortly after Ysmir, in the guise of a storm, visits Hjalti's camp and the two decide to cooperate.
Tiber Septim: The royal name Talos took for himself upon proclaiming himself emperor.

When the Underking gets the Mantella in Daggerfall is when Talos is "created."