20: Honor Thy Mother

The Elder Scrolls IV: Oblivion Guide - Dark Brotherhood

20: Honor Thy Mother

When you're in Bravil with the other members of the Black Hand, Arquen will recite an incantation in front of the statue there, allowing the group to enter the Night Mother's Crypt. Inside the crypt, Mathieu Bellamont will eventually reveal himself as the traitor and quickly kill off two of the Speakers.

Kill the traitor!

He will then attempt to kill the Night Mother. After speaking to Arquen, quickly approach Bellamont and kill him. He's quite weak, and Arquen is there to help, so he should fall fairly quickly. Once he is dead, the Night Mother will approach, and name you the new Listener of the Black Hand! You are then permitted to take the treasure from her crypt. You may find some useful enchanted items, so be sure to search the crypt thoroughly, as you won't be able to enter again once you leave.

When you're ready to leave, speak to the Night Mother again. She will upgrade the Blade of Woe to a much more powerful state. Once back in the Dark Brotherhood Cheydinhal Sanctuary, speak to Arquen there to complete the quest.