Tiber Septim was a military general and fought to unite the land of Tamriel. He ruled over Tamriel for thirty eight years, and started the noble Septim family line, which all Emperors are selected from. Tiber is regarded as one of the Nine Divines, and there is a statue of him in Bruma.
There are two versions of the history, and both are detailed below. Whether they are deliberately different or the developers made a mistake however, is unknown.
Tiber Septim was born 'Talos', in a location called Atmora. According to information present through the Elder Scrolls games, he spent his early life in the region of Skyrim, which was where all Nord races initially hailed from. At the age of 20, Tiber led a battle, where his men reclaimed a village that had been seized by enemies. After this his men retreated, but Talos advanced forwards, until he discovered the cave of the gods in a mountain. When his name was said, the earth supposedly shook, and he was told that he was destined to be the leader of a unified Tamriel.
After this, Tiber took over the area of the Colovian Headlands and ruled it under an emperor by the name of Cuhlecain. He commanded the Colovian army, until he was asked to take part in battle against the people of Skyrim. When he arrived, the people of Skyrim recognised him as one of their gods, and joined force with the Colovian army. Several years after this event, an assassin slit the throat of Talos, as he was still known. While Talos survived this, he lost his voice. Despite this he managed to rule over Tamriel. He adopted the name of Tiber Septim, and also the name of "Ysmir", which is a Nordic name, that translates to "Dragon of the North".
Talos was born on an island in the province of High Rock. During his lifetime, there were rumors of a warlord who would manage to unify all of Tamriel under one nation. Talos rose to a position of power under Cuhlecain, and had a secret ally who he never revealed to his friends. This ally was a Nordic king who had supposedly been dead for many years, called Wulfharth. During critical moments, Wulfharth supposedly took the role of Talos and fought for him, with more ferocity than Talos himself could muster. After the conquering of this province, Talos decided to slaughter Cuhlecain, and assume his position of power. Wulfharth and Talos remained allies through this time.
While Tiber isn't ever clearly and openly encountered in any of the games, as he would be dead for an extended period of time before they took place, Tiber Septim arguably has a major impact on the story, as he was one of the Emperors' ancestor's brother during the games and became One of the Nine Divines (a godly figure named Talos). Even though he isn't openly encountered in any of these games, an interesting man named Wulf appeared in Morrowind. Wulf was encountered by the Nerevarine in tavern near the Ghostgate, just before Second Battle of Red Mountain led by reincarnated Nerevar. Wulf asked the hero if he would carry his old coin around Red Mountain. The coin wasn't looking unusual, it was just very old. It was up to player if he would accept the coin or not. If he did accept this coin, Nerevarine would gain a spell named "Emperor's Luck" which gives a quite powerful, temporary boost on luck. If Nerevarine would ask another people about Wulf, no one would claim that he exists or have seen him, and if he would ask about Wulf in church of the Nine in Ebonheart, the priest would say that the individual was probably an avatar of Talos.