"I have been waiting for you, assassin. We have not spoken before, but I am well aware of your accomplishments within the Dark Brotherhood. That is why I have sent for you. I'm afraid there is a... situation. The time has come to test both your skill and your loyalty to me again." The Dread Father told the hooded spirit before him.
"I will serve you for time immemorial!"the spirit said.
"And that, dear child, is why I have chosen you. The Dark Brotherhood has been destroyed. That is why I have brought you here; you will be sent as Listener to a new world; almost untouched by war and violence."
"It shall be done." The spirit said saluting.
"Go now, child of Sithis, and know that should you fail; your torment will far worse than anything you have ever felt before."Sithis said, as he opened the portal and sent the assassin through it.
...you do realize that while Sithis does exist, he's not the ruthless death god the Brotherhood worship? The thing the DB worship is probably Mephala, although the statue of Sithis in Deepscorn Hollow is probably Lorkhan.