The traitor in the Dark Brotherhood in The Elder Scrolls IV: Oblivion is Mathieu Bellamont. He is a Breton assassin who was once a member of the Cheydinhal Sanctuary but turned against the Brotherhood due to a personal vendetta, seeking revenge for the Brotherhood's killing of his mother many years prior
. During the questline, you discover that Mathieu has been secretly sabotaging the Dark Brotherhood by switching dead drop contracts and orchestrating the assassination of key members of the Black Hand, the Brotherhood's leadership. This betrayal leads to widespread mistrust and chaos within the Brotherhood, and initially, Lucien Lachance, the Speaker, is suspected and eventually killed by the Black Hand under the false belief that he is the traitor
. The truth comes to light in the quest "Following a Lead," where you uncover Mathieu Bellamont hiding in the cellar of the Anvil Lighthouse. His diary reveals his motivations and hatred towards the Brotherhood and Lucien Lachance. Mathieu's treachery causes the player to purge the Sanctuary by killing all its members, as ordered by the Black Hand to cleanse the Brotherhood of the traitor's influence
. In summary, Mathieu Bellamont is the true traitor in the Dark Brotherhood questline in Oblivion.
