Every bit of lore we know about the Dark Urge has been worked into this, with headcanon filling in the sizeable gaps.
From the moment Igarak took his first breath, his ever-faithful butler was at his service. Sceleritas Fel's first task for his little master was finding him an appropriate home. Not just any home! But one that could shape the Murder Lord's progeny into something useful. He chose very carefully.
He found a human couple working in a poorer district in the Lower City, surgeons of great skill but little means who'd spent years trying and failing to conceive. Being charitable, Ilmater-worshipping people, they were eager to adopt--which, to Sceleritas' mind, meant they were pitiful and desperate. They would love the Dark Urge without question. They wouldn't flinch at their little tiefling's horns or his tail or shrink away from his glowing eyes when fixated on them with just a bit too much intensity.
While they didn't have a lot of coin, they had enough to provide a basic education. Not too much--no need to have Bhaal's scion questioning things he shouldn't!--but just enough to ensure he had the skills he would one day need.
They loved him. They named him. (Not really understanding tiefling naming conventions, they thought they were giving him a traditional name. Instead, they gave him an infernal name--but few people ever blinked at a tiefling with an infernal name.)
For the most part, his childhood was normal. He was taught to read and write. More crucially, he learned their skills.
Sometimes, he was allowed to be in the room where they worked, watching them stitch wounds, reset bones, mend pieces of a person's body. He read their books and traced anatomical diagrams with his tiny claws and soaked in every fascinating detail. At first, they encouraged his curiosity. Children were inquisitive! There was nothing wrong with him wanting to learn from his parents, right?
As he grew, though, so did the preoccupation with their craft.
He'd stare a little too long at their surgical knives. His eyes would track the movement of their hands with unnatural focus. He'd ask questions that disturbed them. How long could someone live without a stomach? How much blood loss was too much? Could lightning stop a heart?
When patients died, he didn't ask why but how--how did they die? What did they look like?
That was when they started keeping him away from their work and steering him toward other interests. For a while, it seemed to work. He was delighted to hone the magic he was born with. Morbid curiosities aside, he was still a kid. He laughed and played and showed off magic for his friends. He loved his parents. They loved him. Even as they worried, they loved him.
As time went on, dark, intrusive thoughts continued to whisper to him. Immobilize. Cut. Squeeze. Choke. Watch.
He ignored them. He was a sweet child. He didn't want to hurt anyone. He would never.
One day, when he was ten, he fell asleep. When he woke up, his parents had been hurt. He stood frozen before them, a small dagger in his tiny hand.
Before the blood had dried, a creature emerged from the darkness to quiet his broken sobs. There was no need to cry, the creature told him. His true family would find him one day. He need only follow his heart.
For the next few years, Igarak survived on the streets of the Lower City. His skills in magic and survival were honed by necessity. At eleven, he made his first deliberate kill out of self-defense.
At thirteen, Sceleritas Fel returned and led him into the Bhaal temple hidden beneath the city. There, he learned what he truly was: a child of Bhaal himself. Not just any Bhaalspawn, however--the Bhaalspawn, a being created from the Murder Lord's own essence, destined to bring about the death of the world.
From there, things accelerated. Sarevok and Sceleritas became his tutors. He was educated on the history of his bloodline, trained to kill more efficiently, and forced to murder again and again until no hesitation remained. He learned to give sermons, to perform sacrifices, to deliver swift punishment. He rose through the ranks. With no one to discourage him, and many urging him on, his dark curiosities were sated. He discovered how long someone could live without a stomach (longer than expected). He learned that four to six pints of blood was the threshold for survival. And he learned that, yes, lightning could stop a heart.
A mortal man served as the temple's high primate, until Igarak came of age at sixteen. By then calling himself the Dark Urge, he cut the man down and claimed his rightful place as the temple's leader. A few, foolish challengers arose throughout his first few years, but each was dealt with swiftly. He was named his father's Chosen, feared and revered by all.
All, save for his sister in slaughter, Orin the Red.
At twenty-five, Igarak formed an alliance with Enver Gortash, Chosen of Bane. Years later, they were joined by Ketheric Thorm, Chosen of Myrkul. Together, they devised a plan to enslave the Elder Brain beneath Moonrise Tower using the Crown of Karsus. Its corrupted magic altered the tadpoles the Brain spawned, halting their hosts' transformation into mind flayers until commanded. All was going well--until Orin's resentment came to a head.
Completely blindsided by her betrayal, Igarak was stabbed through the head and left for dead in the colony, a tadpole implanted by Orin gnawing at his brain matter. Barely alive, he was found by the Myrkulite necromancer, Kressa Bonedaughter.
As the first person to survive a tadpole insertion, Igarak was named the first True Soul, although no one else knew about it. Awed and inspired by his ability to survive, she became obsessed with him. In her chambers, she experimented on him, studying the tadpole's effects, vivisecting him, pushing his body beyond what it should be capable of. Igarak was delirious at this time, unable to speak and fading in and out of consciousness. He fought in whatever way he could, once even trying to strangle her with his own intestines, but he was far too weak.
Eventually, Kressa's husband, disgusted by her experiments, betrayed her secret to Balthazar. The experiments ended. Igarak was taken away from her and placed aboard a Nautiloid.
Interested in what happened after that?