Well, someone said his backstory was to common and cliche in the backstory thread once.
Though Daeo was the son of Wendalin Fowl and Rugnar the Terrible. Each of his families bloodline was steeped in cruel forms of death and blood. Wendalin's family side were pervese worshippers in evil Deadric prisoners and of Sithis himself. Many of his ancestors were Dark Brotherhood assassins. Where as Rugnar the Terrible were necromancers, who once took a hold of a small farming village by themselves with powers of undead and mind bend. It ended up in the smallest, but bloodiest "war" between High Rock Guards and cult Necromancers.
He has an older sister, and I have thread floating around about her somewhere. Basically, every random "Go Kill a Beggar" and "Go Kill this Stormcloak Lady" is the work of his older sister Felicia.
Felicia herself has some brutal tales in Skyrim herself.
Felicia is bitter and jealous of most woman, despite her beauty transcending anything, but godly. [though the dark makeup and the war tattoo that I used to enchance her eyeliner] don't help.
She lures people with her Illusion spells and seductions out to where she lives, to use them as sacrifices for the Black Sacrament. Quite literally becoming Skyrim's own Night Mother. As the one now rotting in a coffin, is from Cyrodil really.
She also kills indescriminantly, her moods swinging from sorrowful to lashing out and angry.
Her cruelest kill was that of Anise, the little old woman, whom she took it upon her to steal her house. [I know technically in game, it can't be done] But Felicia raised Anise from the dead, so she could become a loyal servant to her. Cleaning and cooking in the very house she lived in that Felicia stole from her, unable to do anything about it.
Felicia probably should be more feared than Daeo. And even Daeo should watch out and probably never meet up with his older sister.