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.