There existed in ancient Finland two clan, each led by one of two brothers: Kalervo and Untamo. They didn't get along and constantly entered into conflict with each others. One day, Untamo snapped. He massacred almost all of Untamo's tribe, forcing the survivors to flee. One of them was Untamala, Kalervo's bride-to-be. She was a quite powerful sorceress and decided to organise her revenge. She made a homunculus from her and Kalervo's hair whose sole purpose would be to carry on their legacy and getting revenge for the clan. Thus was born Kullervo, Son of Evil. However, she had to flee from Untamo's soldier shortly after he was finished, leaving him alone. When he was brought before his "uncle", the latter was shocked by the infant's declaration that he would kill the massacrer of his clan. Untamo tried to execute him, but nothing managed to affect the child. He then raised the boy as a slave, hoping that all of the constant work would distract him from his revengeful toughts. However, the artificial human used his magic whenever possible to undermine his slaver. Desperate, Untamo sold him to the blacksmith Ilmarinen shortly after the young reached adulthood. The blacksmith employed him as a shepherd and mostly didn't bother him that much. The real problem was his master's wife. For some reason known by her alone, she delighted in making the young man's life as miserable and difficult as possible. She fed him pastries filled with rocks, gave him absurdly-long prayers to recite and even broke his most cherished possession. This last act brought him over the edge and he used his spellcraft to kill her and escape from this life. He was resulate to find Untamo and make him pay.
However, while he was travelling, he heard a most baffling informatio: Kalervo and Untamala were alive and founded a new community ! Incredulous, he searched and indeed found them. They told him that they found each other, married at last and founded a familly. Untamala had forsaken her desire for revenge and just wanted to raise the son she couldn't be here for. For a time, Kullervo tought to himself that he had no reason to seek revenge anymore and that he could just live peacefully with his newfound familly. Yet it was not to be. He quickly found out that his strange upbringing and his overwhelming strength made all social interactions and work all almost impossible. Desperate to find him a role, his adoptive father send him to pay the taxes their village owed to the current ruler of Finland. Also desperate to find a place for him in this society, Kullervo eagerly filled the task. On his way back, he met a beautiful maid. They fell in love and even had carnal relations. Afterwards tough, they discovered a awful truth: the maid was the daughter of Kalervo and Untamala who they lost. Drowning in shame, she killed herself.
This last misfortune was the point of breaking for Kullarvo. He realised how foolish he was for seeking to live a normal life. How could he, when he was not a man, but the incarnation of a days-old revenge that has long be forgiven ? Whose has no place to call home, no people to truly call familly ? He then decided to fulfill his only reason to live: getting revenge on Untamo. He went back to his... no, the home of his parents to tell them what he has done and what he would do. They were understably horrified at his act of bedding his sister. More than that, they tried to make him renounce his plan, saying that it would only bring him misery. Their arguments quickly degenerated, and the foolish homunculus blew his familly members off, saying that he would not care when they died, and that he could just replace them with others homunculus of his own. He then set off, ignoring whatever replies they might have. On his way to the place where Untamo reigned, he heard from messengers that his familly members died. He initially acted aloof, saying that their death wasn't important, until he heard that his beloved mother died. He then wept and cursed. He cursed the gods, for making his life so miserable. He cursed his mother, for making him and abandoning him in this cruel world. He cursed Untamo and the blacksmith's wife, for robbing him of his childhood. He cursed his sister for making him fall in love with her. But more than that, he cursed himself, for being so stupid. With a newfound resolution found from despair, he demanded a sword to the god Ukko, who granted him his gift and attacked Untamo's tribe. They were no survivor. He killed warriors and civilian, men and women, elderly and children, noble and peasants. He didn't stop until they were no longer any victim to kill. But even so, he found no satisfaction in the act. His familly wouldn't come back and, now that he fulfilled his purpose, he no longer had any reason to live. He then heard his mother's voice, who told him to go live with the nymps. He rejected the offer, and accepted the one proposed by his sword, far more seductive considering his circumstance. The sword then freed him from the misery of live. Thus died those who would later be called the Son of Evil.