Mathieu Bellamont
From the OblivioWiki – your guide to everything Oblivion
Mathieu Bellamont is a speaker of the Black Hand. As a boy he observed the murder of his mother from underneath his bed, and the sight of her severed head apparently drove him to insanity. He learned of the Dark Brotherhood's involvement and joined their ranks for the sole purpose of killing his mother's murderer, Lucien Lachance, and later the Night Mother. He conspired to kill every member of the Dark Brotherhood, murdering some himself in the crypt of the Night Mother, but when he was nearly caught in the act he decided to use the unknowing Silencer of Lucien Lachance for that purpose via intercepted dead drop letters.
Operating out of the Anvil Lighthouse basement, he plotted against the Brotherhood while gratifying his insatiable homicidal need. Leaving a host of dead bodies, from men and women to rats, dogs, and other creatures; generating a terrible smell noticeable to passers-by. Apparently torturous, as a lone dog is found in the cellar, driven mad. He keeps the severed head of his mother in a shrine there (Which can, if you wish, be picked up, but there's no point really, it has about the same value as a spoon, it DOES serve as an interesting Easter Egg though, if you pick it up, and bring it to Applewatch where Mathieu and the others are at just after Lucien Lachance is murdered and you are promoted to Speaker by Arquen, drop it in front of Mathieu, then talk to him when it is on the ground, he will stutter, and act confused and surprised, and claim he is distracted and end conversation with you, while none of the other Speakers even make notice of it).
In his journal he states that he killed the crew of the ship The Serpent's Wake who became vengeful spirits. He revels at the idea of a famous ghost ship that he created. He was apparently provoked by the crew's taunts of "rat human". He also despairs about a woman he loved named Maria, another member of the Brotherhood, who he seems to have killed in anger. Based in the context of his diary, Bellamont hints that Lucien Lachance may be his father, however, Bellamont is also mentally unstable so this could just be included for poetic means.
