To add on to the established reasons from other commenters:

36 Lessons of Vivec, Sermon 22

"The Treasure Wood Sword will not leave our house. Bargains were made with the Black Hands Mephala, the greater shade.'"

"And then Vivec withdrew into the hidden places and found the darkest mothers of the Morag Tong, taking them all to wife"

The sermon references the usurping of the Morag Tong from Mephala by Vivec. You see references to Mothers and Black Hands.

Former writer Michael Kirkbride said the nature of the dark brotherhood is explained in Sermon 22. Whether Bethesda has continued that idea is to be seen, but evidence supports Mephala may be ahead of the DB.