The Slave Boy whose birthname is unknown, but who later renamed himself Shaitan, is referred to as the founder of the Baali line.
There are rumors that the origin story of the Slave Boy is a fabrication by Nergal, another Fourth generation Baali who comitted similar atrocities, but came into being a long period of time later. You might also check the following links for more information on the subject:
A childe of the mythical Antediluvian Ashur, the slave who was to become Shaitan had been embraced for his great beauty and a coercing voice, which was unrivaled among his brethren of the Fourth Generation. The slave , however, grew jealous of his kin, mainly the Toreador Antediluvian and Zillah. Bitter over the fact that he would always be unequal to those of lower Generation, he sought to become a power in his own right. When the Third Generation revolted against the Second Generation in the First City (note that this is incompatible with other noddistic myths, in which the revolt takes place in the Second City), the slave gathered the other dissatisfacted of all Clans and laid siege to Caine's palace himself, cursing the gods to whom he once had sung hymns. Seeking powers from the forces of Hell, the slave sought to slay Caine himself, but was cast down and judged by the First Vampire for his transgressions. Caine stripped him of his beauty and his powers, leaving him near-death. The disciples of the slave Boy mourned and carried him away.
The Slave Boy, however, had retained the charm of his voice and cried out to the forces of Darkness to save him. He was answered and remade into a being of greater power, named Shaitan. He asked of his disciples to infiltrate the thirteen Clans, while he retreated to a fortress in modern Israel, called Chorazin. There, he plotted to take his revenge on the world by summoning his masters into this realm and rule the remains of the world as their seneschall. When the stars were right, he moved his place of power to Crete, into the city of Knossos, where he erected the labyrinth as a focus of infernal energies. He had, however, underestimated the other Clans, who gathered to stop him and eventually shattered the labyrinth in a ritual conducted by the Followers of Set.
Since then, Shaitan had vanished and was thought to be destroyed. Some Baali, however, claim that he communicated with them in dreams, telling them that he had fled to a distant land to gather his strength.
- The Dark Ages Companion, p. 170