To Serve Sithis is a quest in The Elder Scrolls IV: Oblivion. It begins immediately upon speaking with Vicente Valtieri and completing "Scheduled for Execution" and ends as soon as the Hero of Kvatch accepts his next contract as part of "The Assassinated Man."
After completing the previous quest, accept another contract from Vicente Valtieri at the Dark Brotherhood Sanctuary.
I must accept a quest from Vicente Valtieri.
I have accepted a quest from Vicente Valtieri.