|Related Quests||Following a Lead, Honor Thy Mother|
Spoiler: End of Dark Brotherhood
It turns out that Mathieu Bellamont is the true traitor. When you enter the tomb of the Night Mother, he kills off most of the members of the black hand and then tries to attack the Night Mother. However, she cannot be killed. As soon as he realizes this, he turns on you. You should be able to defeat him easily.