Board Thread:Lore Discussion/@comment-24640703-20140419210003/@comment-13446185-20140419210727

The Night Mother is Mephala.

''The Night Mother, my dear friend, is Mephala. The Dark Brotherhood of the west, unfettered by the orders of the Tribunal, continue to worship Mephala. They may not call her by her name, but the daedra of murder, sex, and secrets is their leader still. And they did not, and still do not, to this day, forgive their brethren for casting her aside.''

-Fire and Darkness

This is supported by Kirkbrides statement as Sermon 22 speaks of Mephala and the Morag Tong.