Bel was a minor Goa'uld who served several System Lords in his career. Initially favoured by Marduk, Bel encouraged rebellion among his master's people before fleeing to serve under Ra. After Ra's death, Bel took his forces into the service of Apophis, narrowly escaping the supernova which killed the System Lord a few years later. After learning about Zigara, Bel began to search for the world which, he believed, would allow him to draw power to himself and claim his own place among the System Lords.
Service to Marduk
Born to the name of Hamar, Bel was naught but the last in a long line of Goa'uld who became the favoured of Marduk. As such, he was given the title Bel Merodach, meaning Lord of Marduk, to serve as Marduk's underling. Although Marduk's forces, much like any other Goa'uld's, included a large number of Jaffa, he was known to eschew the use of First and Second Primes in favour of appointing one of his own 'sons' to do their role; Bel was one such Goa'uld. In this role, Bel became privy to the fact that the System Lords lost their reasons to support Marduk, and indeed even his own priests began to balk at his actions. As such, Bel encouraged open rebellion within Marduk's people and even supplied them with the fitting means to punish Marduk.
Following this revolt, Bel chose to flee to Ra-controlled space along with a few of his trusted associates, dropping the Merodach part of his title in order to build up political power of his own. Due to the fact that Marduk's reputation was still fresh among the minds of the other Goa'uld, Bel was largely left alone by other ambitious leaders meaning that his efforts to build up contacts of his own were often thwarted. Nevertheless, Bel did manage to grab a few worlds of his own—mostly from his 'fathers' old territory—and was occasionally able to recruit Marduk's former underlings to his cause. None of these remained for long, however, and even Surpanakha—a Goa'uld in whom both he and his father placed much of their trust—chose to unwisely seek out the secrets of Cimmeria, eventually freeing her host Kendra.
As such, Bel's ascension through Ra's ranks was soon stalled, until help came from an unlikely source. When the Tau'ri killed Ra over Abydos, Bel chose to defect his loyalties to the now-powerful Apophis, taking much of his forces with him. While he was still not regarded to be as important as he had in his 'fathers' court, Apophis did recognize his worth and gave him the role of a minor Lieutenant. Zipacna, one of Apophis' other underlings, did not consider Bel worthy of the position he had taken and so an enmity and rivalry soon resulted.
Within only a few years, however, Apophis' forces were all but destroyed in a nova above Vorash. Bel, being familiar with the signs of an imminent supernova from his training under Marduk, seized this opportunity and escaped the area in his Ha'tak with only minimal damage. Making the way to worlds which he had been given nominal control over under Apophis' name, he bided his time and made repairs to his ha'tak until word came that Apophis was truly dead. One such planet was Marduk's zoo, which Bel had turned into a training world for his new troops soon after Marduk's death. With this planet, Bel was able to train elite troops to serve in his name.
The search for Zigara
It was because of this planet, along with a few others, that Bel was in a position to react when the Zigarans appeared through a Stargate in his territory. After interrogating the Zigarans whom he had captured, Bel realized the importance of Zigara and soon set about to reclaim it in his name, hoping that the planet would consolidate his power and allow him to claim the rank of System Lord. Bel's initial attack was met with surprise by the locals, and as such he was able to take swift control of the caverns surrounding the Stargate. It was only when the Zigarans broke out their experimental energy weapons that Bel's advance was stopped. A stalemate resulted from this, though the Zigarans knew that Bel would either break through their defenses or locate their planet and send ships. (RPG: "Living Gods: Stargate System Lords")