Bhaalspawn

Over the years, Baldur's Gate has harbored countless evil schemes and conspiracies. Perhaps the most dastardly, though, was that perpetrated by Bhaal, god of murder, who foresaw his own death during the Time of Troubles. While in mortal form, Bhaal conceived multitudes of offspring, plotting to have them resurrect him. They actively hunted and slew each other, with the survivors collecting ever more of Bhaal's godly essence.
Sarevok, a Bhaalspawn and member of the Iron Throne merchants' guild, sought to spark war with the nation of Amn and become the new Lord of Murder. Another Bhaalspawn, aided by powerful allies, thwarted Sarevok's plan and brought stability, if not peace, to Baldur's Gate once more. Yet, Bhaal and his faithful have menaced the city ever since.

History

It was said that Bhaal foresaw his death during the Time of Troubles of 1358 DR and came to Toril before the climactic event, mating with females of almost any species, not all of them humanoid. Like Bane, he attempted to use these offspring to bring himself back to life. Alaundo made a prophecy about the Bhaalspawn that foretold the great destruction they would bring.

Nature

Bhaalspawn mostly seemed like normal members of their race at first. Part of their souls were divine, however, and this would eventually reveal them, sometimes granting supernatural powers of different kinds but usually also tempting them to evil and murder. Individual reactions were very different, however. The personalities of individual Bhaalspawn were just as varied as those of other people, but they all had to deal with similar issues when their other side manifested itself. Some, such as Sarevok and most of the Five, fell to the temptation of power and embraced their darker sides in a quest for divinity.
Not all Bhaalspawn were evil. Some had other ideas than gaining power, had no such ambitious goals and might have even tried to do what was right, but reveled in bloodshed nevertheless. Yet other Bhaalspawn, such as Balthazar and Gorion's Ward, conquered their darker nature and acted selflessly, turning their divine power into something unlike its original nature. Finally, some Bhaalspawn hadn't even had to deal with their inner taint before being found out and being persecuted or taken advantage of because of it, like Imoen and some of the Bhaalspawn in Saradush. Some of the latter were simply ordinary people who wished no part in the prophecy. And yet, it was that very prophecy that predicted that Bhaal's children should be not only of all races but also of all alignments.
If Bhaal's plans had gone as they were meant to, the Bhaalspawn would have destroyed each other until none remained, at which point the essence of Bhaal scattered among them would have accumulated, and his chosen follower, Amelyssan, would have performed rituals that would have brought him back. Neither Amelyssan nor the last Bhaalspawn complied with these plans, however.
As a side effect unintended by Bhaal, the Bhaalspawn's share of the essence meant that each of them had the potential to become a god in their own right. However, they had to acquire the rest of the essence. If successful, Sarevok, who tried to fulfill the prophecy, would have simply have turned into a resurrected Bhaal. Another side effect was that it was theoretically possible, if rare, for a Bhaalspawn to turn into an avatar of Bhaal, either the Slayer or in more extreme cases the Ravager (both of which appeared as creatures far more bestial than the original avatars).