The Elder Scrolls: The Secrets Of The Dark Brotherhood
With the onset of the Dragon Crisis, years after the death of the last Listener, Cicero, the Keeper, takes the Night Mother's coffin to Falkreath, in Skyrim, where the Dark Brotherhood sanctuary was led by a Nord named Astrid. In the absence of the Night Mother, Astrid holds the responsibility of finding new recruits for The Dark Brotherhood, shaping her guild with much more modern values of family and unity, and throwing shade to the five traditional laws that govern them.
When Cicero arrives with The Night Mother, the The Dragonborn joins their ranks, emerging as the new Listener. Astrid grows jealous - her leadership threatened - and betrays the Brotherhood. Falkreath Sanctuary is attacked and most of its assassins are slaughtered. Astrid knowingly breaks every single one of the Five Tenets, and she pays dearly for it. She ends up begging for her own death, even going as far as to perform the Black Sacrament on herself. Such, it seems, is the wrath of Sithis.
It is theorised that The Night Mother is actually the daedric prince, Mephala. There are reasons to suggest that this could be true - that the corpse buried in the coffin is just that, a corpse, and the voices the Listener hears in their head are that of Mephala. Mephala was the daedric prince of deceit, and the figurehead behind the Dark Brotherhood's mortal enemies, the Morag Tong, so it is believable that Mephala would pose as the Brotherhood's figurehead, and laugh cruelly at their blind obedience to a mere corpse, perhaps leading the guild down a gradual path of self-destruction.
It is also theorised that The Night Mother, The Dark Brotherhood, and to an extent, Sithis, are all being orchestrated by the daedric prince of madness, Sheogorath. Sheogorath is described as "The Sithis-shaped hole of the world" in myths, linking him to The Dread Father. He revels in the act of driving mortals insane, or making them perform trivial, silly tasks. He would find the whole thing hilarious, watching this very dark, foreboding, serious cult carry out their business, if he was really the one whispering through The Night Mother's corpse the whole time.