Talos is, and yet is not, Tiber Septim. Rather, Tiber Septim was an identity worn by both Hjalti and Wulfharth/Ysmir. While Hjalti did receive the name 'Talos' (Stormcrown) in life, it was because people thought Ysmir's power was Hjalti's. Talos is essentially Shor/Shezzar/Lorkhan "reborn". Or perhaps more accurately: Talos is an echo of an echo.
Says one source of questionable accuracy.
Even so, the Heresy is an important book.
The Divine Crusader is the Shezzarine; he could not destroy Umaril before, but later, Talos granted this power to destroy Umaril to the new Shezzarine.