I don't really consider Talos one of the Divine as he is just Tiber Septim on a pedestal.
Yeah, I would have preferred that as the actual explanation, but they seriously did bend the lore over backwards to explain the deification of Tiber Septim into Talos. Talos is apparently an amalgamation of 1, 2, or 3 different entities depending on how you look at it. There are 3 basic entities, whic are Tiber Septim, King Wulfharth, and Zurin Arctus. Tiber Septim is the chief member among them, being the Emperor. He is also supposed to be the Shezarrine, the avatar of the god Shezarr. However, Wulfharth is also supposed to be a Shezarrine, but at the time he is an undead being known as the Underking, which means there are two Shezarrines walking around. But then Zurin Arctus is
also supposed to be the Underking, but is also Tiber's chief battlemage. His heart was used to power the Numidium, one of the reality-defining towers of the world, and using that, Tiber achieved godhood. By doing so, however, all three of them became involved and were simply merged into the entity known as Talos.
King Wulfharth is known as Ysmir. He is a Shezarrine. He is the Underking.
Tiber Septim is also known as Ysmir. He was known as Talos before ascending to godhood. He is also a Shezarrine. He is the primary interpretation of the god Talos
Zurin Arctus was a Battlemage. He is also the Underking. His heart was used to power the Numidium which created Talos.
The god Talos is also known as Ysmir,
My interpretation is this:
A Shezarrine is an avatar of the god Shezarr, also known as Lorkhan. Wulfharth was an earlier Shezarrine, but he died. Afterwards, Tiber Septim came around and he was the latest Shezarrine. But at some point, the dead Wulfharth was brought back to life as the undead Underking. Tiber Septim planned to activate the Numidium, but it needed a power source. One story states that Tiber and his Battlemage, Zurin Arctus, tricked Wulfharth into giving up his life force to power the Numidium. Yet the physical object powering the Numidium was Arctus' own heart. What could have happened is that Arctus' heart was used as a soulgem, and they acquired the soul of Wulfharth by somehow soul trapping him into Arctus' heart. This way, Arctus then became the Underking. Arctus was able to persist without a heart because he gained Wulfharth's undead status, and still retained his own soul. But, Wulfharth is a Shezarrine. By soul trapping him, they actually soul trapped Lorkhan himself, which means they also soul trapped Tiber Septim who was also a Shezarrine. In the end, the Numidium helped Tiber ascend to godhood, and he brought Arctus and Wulfharth along for the ride who became attached to him on a metaphysical level. Thus was born the deity Talos, who is comprised of all three of them and substantiated by the power of Lorkhan.
It's entirely possible that Lorkhan himself was turned into Talos, but the mechanics of the world state that once someone is made into a god, they persist for eternity. Even if you are turned into a god afterwards, it is as if you have always existed, like Arkay and Mannimarco. Lorkhan is dead, yet he persistently exists, and though Lorkhan could also be converted into Talos through his avatars, Lorkhan in his original form would still persist. A god out of a god.
The main problem is that it's never actually stated how he gained godhood, though most sources imply that the Numidium was the cause. I guess it's the most likely choice, given that its status as a "Tower" (one of the important towers) allows it to bend reality. If he used the Numidium to ascend to godhood, then that would explain how Wulfharth and Arctus also comprise the god Talos.
Long and complicated, and very unnecessary. This would all be made less difficult if they just outright said that Tiber Septim is a false god like Julius Caesar...