Hmm, I am not sure it is in one of the in-game books but here is a bit of
FR lore about how Bhaal became Bhaal.
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HISTORY OF THE DEAD THREE:
'Knucklebones, Skull Bowling, and the Empty Throne'
In ages past there was but one god of strife, death, and the dead, and he was known as Jergal, Lord of the End of Everything. Jergal fomented and fed on the discord among mortals and powers alike. When beings slew each other in their quest for power or in their hatred, he welcomed them into his shadwy kingdom of eternal gloom. As all things died, everything came to him eventually, and over time he build his power into a kingdom unchallendged by any other god. Eventually, however, he grew tire of his duties for he knew them too well. Without challenge there is nothing, and in nothingness there is only gloom. In such a state, the difference between absolute power and absolute powerlessness is undetectable.
During this dark era, there arose three powerful mortals - Bane, Bhaal, and Myrkul - who lusted after the power Jergal wielded. The trio forged an unholy pact, agreeing that they would dare to seek such ultimate power of die in the attempt. Over the length and breadth of the Realms they strode, seeking powerful magic and spells and defying death at every turn. No matter what monster they confronted or what spells they braved, the three mortals emerged unscathed at every turn. Eventually the trio destroyed on of the Seven Lost Gods, and they each seized a portion of his divine essence for themselves.
The trio then journeyed into the Gray Waste and sought out the Castle of Bone. Through armies of skeletons, legions of zombies, hordes of noncorporeal undead, and a gauntlet of liches they battled. Eventually they reached the object of their lifelong quest - the Bone Throne.
"I claim this throne of evil", shouted Bane the tyrant. "I'll destroy you before you can raise a finger", threatened Bhaal the assassin. "And I shall imprison your essence for eternity", promised Myrkul the necromancer.
Jergal arose from his throne with a weary expression and said, "The Throne is yours. I have grown weary of this empty power. Take it if you wish - I promise to serve and guide you as your seneschal until you grow comfortable with the position". Before the stunned trio could react, the Lord of the Dead continued: "Who among you shall rule?"
The trio immediately fell to fighting amongst themselves whiled Jergal looked on with indifference. When eventually it appeared that either they would all die of exhaustion or battle on for an eternity, the Lord of the End of Everything intervened.
"After all you have sacrificed, would you come away with nothing? Why don't you divide the portfolios of the office and engage in a game of skill for them?" asked Jergal. Bane, Bhaal, and Myrkul considered the god's offer and agreed. Jergal took the heads of his three most powerful liches and gave them to the trio that they woud compete by bowling the skulls. Each mortal rolled a skull across the Gray Waste, having agreed that the winner would be the one who bowled the farthest. The trio immediately fell to fighting amongst themselves whiled Jergal looked on with indifference. When eventually it appeared that either they would all die of exhaustion or battle on for an eternity, the Lord of the End of Everything intervened. "After all you have sacrificed, would you come away with nothing? Why don't you divide the portfolios of the office and engage in a game of skill for them?" asked Jergal. Bane, Bhaal, and Myrkul considered the god's offer and agreed. Jergal took the heads of his three most powerful liches and gave them to the trio that they woud compete by bowling the skulls. Each mortal rolled a skull across the Gray Waste, having agreed that the winner would be the one who bowled the farthest.
Malar the Beastlord arrived to visit Jergal at this moment. After quickly ascertaining that the winner of the contest would get all of Jergal's power, he chased off after the three skulls to make sure that the contest would be halted undtil he had a chance to participate for part of the prize. Bane, Bhaal, and Myrkul again fell to fighting as it was obvious their sport was ruined, and again Jergal intervened. "Why don't you allow Lady Luck to decide so you don't have to share with the Beast?" The trio agreed, and Jergal broke off his skeltal finger bones and gave them to the players. When Malar returned from the chasing the skulls, he found that the trio had just finished a game of knucklebones. Bane cried out triumphantly, "As winner, I choose to rule for all eternity as the ultimate tyrant. I can induce hatred and strife at my whim, and all will bow down before me while in my kingdom".
Myrkul, who had won second place, declared, "But I choose the dead, and by doing so I truly win, because all you are lord over, Bane, will eventually be mine. All things must die - even gods."
Bhaal, who finished third, demurred, "I choose death, and it is by my hand that all that you rule Lord Bane will eventually pass to Lord Myrkul. Both of you must pay honor to me and obey my wishes, since I can destroy your kingdom, Bane, by murdering your subjects, and I can starve your kingdom, Myrkul, by staying my hand". Malar growled in frustration, but could do nothing, and yet again only the beasts were left for him.
And Jergal merely smiled, for he had been delivered.