NOTE: adapted directly from the BG3 Wiki with some edits.
Lucasta, the Dark Urge, is a Bhaalspawn, but unlike other Bhaalspawn, she wasn't born – she was created from the dead flesh of Bhaal himself.
Lucasta grew up outside the Cult of Bhaal, and was at some point taken in by a couple in Baldur's Gate. Before long, sensing something was off about their foundling, they surrendered her to the Church of Lathander at Rosymorn Monastery, sure that being raised in the light of the Morninglord would fix her.
She had a relatively peaceful childhood for a time, growing up with a few other children at the monastery. Although she was typically outgoing and playful, the monks noted her seemingly pathological avoidance of service and several incidents involving dead animals on the grounds that were never resolved. Even in her chores, mishaps seemed to befall both her and the monastery. The elder monks encouraged Lucasta to expend her energy training in defense of Rosymorn and the Blood of Lathander if she couldn't sit still and focus on prayers.
Combat training did little to help the secret, deadly urges in Lucasta's brain, but it at least gave her something to focus on to release some of her tensions. She was a skilled athlete and fighter, and in her determination to overcome her horrific imagination, she pledged herself as a paladin.
When githyanki warriors descended on the monastery, Lucasta was eager to help defend her home with the other warrior-monks. In the bloodshed, though, something in her snapped - she succumbed completely to the dark urge that had plagued her all her life and killed indiscriminately. Her paladin oath broken, the sacred sword she had been using shattered in a ray of blinding light, sending a searing pain through her left eye and permanently branding her.
Covered in the blood of her former brethren, she wandered into the wilderness, following her dreams of Bhaal towards Baldur's Gate. She was welcomed into the Cult of Bhaal with open arms, breezing through her initiation with the Murder Tribunal. Her story of revelation gained so much attention that she herself came to be known as the very thing that had saved her - the Dark Urge.
Eventually, as Bhaal's own flesh and blood, the Dark Urge became the leader of the Cult of Bhaal and pledged to end all life in existence for her father Bhaal. At some point fellow Bhaalspawn Orin the Red joined and became her second-in-command.
Enver Gortash contacted the Dark Urge after reestablishing worship of Bane, and the two formed an alliance upon discovering common goals. They were visited in their dreams by Bhaal and Bane, who named them their Chosen and commanded them to ally with Ketheric Thorm, the Chosen of Myrkul. Gortash and the Dark Urge hatched a plan to steal the Crown of Karsus in a heist at Mephistopheles' Vault by having Helsik open a gate to Mephistar for them.
After the heist was successful, Gortash suggested that they use the Crown on the elder brain located in the illithid colony below Moonrise Towers. The Dark Urge agreed to the plan so that she could fulfil her pledge to her Lord: She would create an army of soulless mind flayers, and with the help of this army she would exterminate all life — including, as the last being alive, her own. Fully intent on eventually betraying the other Chosen, the Dark Urge joined Enver Gortash and Ketheric Thorm at Moonrise Towers.
At Moonrise Towers, the three Chosen initiated a raid on the mind flayer colony, and successfully used the Crown on the colony's elder brain, enslaving it to their will. At some point during this, the elder brain came to respect the Dark Urge and intended to go along with her plan, impressed by her unmatched cruelty.
For a time, the Dark Urge was treated like a goddess at Moonrise Towers. But after putting the Crown on the elder brain, she was soon betrayed by her sister Orin the Red, who was jealous of Bhaal's favouritism.
The Dark Urge had her skull pierced and opened with a dagger, and was infected with a tadpole that began to eat her liquidating brain, resulting in her amnesia, as well as her becoming the first True Soul. However, as the tadpole nestled deeper into the darkness in her mind, it was the darkness that grew weaker, that she began forgetting. Lucasta - the Lucasta who was branded with goodness on her countenance - was all she could remember.
Lucasta was found by Kressa Bonedaughter who, impressed with her willpower and apparent resistance to the elder brain's influence, began to study her. She would cut Lucasta open and put her back together repeatedly over the course of several days in the mind flayer colony, during which the part of her that was Lucasta would all but disappear, and even the tadpole-infested Dark Urge who gained control once more was unable to speak or do anything but attempt to inflict violence on herself or others. Noticing his wife's obsession and hoping to snap her out of it, Kressa's husband, Maghtew Budj, (AN: why did they spell his name like that wtf) arranged for the broken and amnesiac Lucasta to be sent off on the Nautiloid Ship, where she would soon wake up with few memories of herself, and none of the Dark Urge.