Bhaal Worship

The folk of Faerûn don’t normally pray to or acknowledge Bhaal. He is seen as a deeply evil and destructive deity who hungers for death — meaning the death of any sentient beings through unlawful means.
Some people pray to Bhaal when they want to commit murder. A person might have good reason to resort to murder, such as when one is unable to redress some injustice through lawful means. But it’s far more common for prayers to Bhaal to be uttered by those who seek to kill someone out of jealousy, greed, or wrath. It’s rare for anyone but assassins or compulsive killers to take Bhaal as a patron, and clerics who revere Bhaal often qualify on both counts.
Murder cults of Bhaal have arisen in the past, each led by a charismatic, self-styled priest of Bhaal, but organized worship of the Lord of Murder is extremely uncommon. Temples and shrines are similarly rare. Those who erect a shrine to Bhaal usually do so to thank him for a successful murder. Such shrines typically feature a skull or a severed head surrounded by drops of blood (often both from the murdered victim).

Divine Origins

Mortal

Before his ascension to godhood, Bhaal was a power-hungry adventurer on Toril. Along with his companions Bane and Myrkul Bey al-Kursi, he sought to attain the portfolio of the God of the Dead Jergal. After defeating one of the Seven Lost Gods, the three were able to travel to Jergal's domain, the Castle of Bone in the Gray Waste. Upon their arrival, Jergal willingly offered his realm to the Dark Three, though they couldn't decide amongst themselves who would rule.
Upon Jergal's suggestion, the three divided his power, deciding how to divide it based on the outcome of a game. The three played a game of knucklebones, and Bane emerged as the victor. He claimed the domains of hatred, strife and tyranny as his own. Myrkul, coming second, chose rule over the dead, the ultimate fate for all of Bane's minions. Finally, Bhaal chose the divine province of death. As he said:

I choose death...I can destroy your kingdom Bane, by murdering your subjects, and I can starve your kingdom, Myrkul by staying my hand.

Godhood

On Eleint 16, 1358 DR, having lost his powers and being forced to walk Faerûn like the rest of the gods, Bhaal was slain by the upstart mortal Cyric using the avatar of Mask, a sword named Godsbane. In doing this, Cyric stole Bhaal's divinity and portfolio elements. However, much like Myrkul—who invested his divine essence in the artifact known as the Crown of Horns—Bhaal was not utterly removed from Faerûn. Part of his divinity remained in the Winding Water, around Boareskyr Bridge where he was slain, his blood having flowed into the river. More importantly, Bhaal foresaw his death and impregnated many mortal women, creating his heirs, the Bhaalspawn. The Bhaalspawn were involved in a series of conflicts along the Sword Coast, with one standing above the others and ultimately foiling Bhaal's plan to return through his children.
After the death of the Lord of Murder, the city-dwelling Bhaalists quickly converted to the worship of Cyric, whom they referred to as Cyric-Bhaal. The rural priests maintained their faith for years to come, claiming they continued to receive their divine powers following their nightly prayers. The schism between Bhaal's remaining followers and the emerging Cyric-Bhaalists reached its apex in 1367 DR, getting to the point where the factions would ambush and assault one another. Soon later, the remaining worshipers of Bhaal stopped receiving their divine powers and began gradually converting to Cyricism or Xvimism.

Reemergence

For a time, it was believed that any possibility for Bhaal's resurrection had been stopped. The last known Bhaalspawn, Abdel Adrian, resisted the murderous impulses caused by his lineage and became a famed and beloved figure in the city of Baldur's Gate. In 1482 DR, near the beginning of the era known as the Second Sundering, Viekang, another Bhaalspawn who was thought to be dead, attacked Adrian as he spoke to a crowd in the portion of town known as The Wide. While the ultimate winner of the duel is unknown, the victor transformed into a massive, blood-soaked creature and began a rampage that was only stopped when a group of adventurers new to Baldur's Gate defeated and killed the monstrosity.
With all of his children dead, all of Bhaal's essence was freed, allowing for his resurrection. Bhaal was revived, and reclaimed the murder domain from Cyric.

Priesthood

The clergy of Bhaal in Faerûn were known as Bhaalists or Bhaalyn, the latter being used more commonly in the lands east of the Dragon Reach. Together they were an disorderly network of local hierarchies, with the urban and rural branches maintaining distant relations from one another. Collectively, they believed that murder was both a duty to their god and a game for their enjoyment. Each cleric of Bhaal was expected to perform at least one murder every tenday, in the darkest moment in the dead of night.
During the time that he was dead, Bhaal had many scattered cultist groups, the most infamous being the Deathstalkers of Bhaal, who attempted to bring him back to life. For a while after the Bhaalspawn crisis, it was believed his essence had been locked away on the Upper Planes, preventing his resurrection permanently. However, this proved to be incorrect. Bhaal's essence had been preserved in his half-mortal children, the Bhaalspawn, and after the death of the last two Bhaalspawn, Bhaal was revived.