It's a need I've come to acknowledge.

My character is a vampire and the Listener.

Then I thought, "well, she'll be the last Listener the DB will ever have".

But then it occurred to me: The next games should still feature this faction.

There's gonna be a conflict since she obviously won't be occupying the position at that point.

So I need to figure out how she could die before those events.

But it's hard to imagine how a normal person would kill an experienced Dragonborn, wielder of several and powerful Shouts, not to mention a Vampire Lord and master of conjuration.

I considered taking the Lord Lovidicus route. Falls in love and gets backstabbed.

But even if her husband locked her somewhere and set the place on fire that wouldn't be enough. I'm sure she could blow away the flames with Unrelenting Force or change into VL and use Mist Form. Or summon two dremoras and have them destroy the door.

The Night Mother speaks her name and she must kill herself in the name of Sithis.

I'd go with suicide.

It's what I went with. Sorta. It was unsuccessful since I wanted to use my character in TES VI in some way(it did put him out of commission for a long time, though), but I see it as the only way to go for a powerful undead creature.

Even the most powerful are not immune to accidents or disease...

The most powerful vampires are, in regards to the disease.

Say she went to Akavir and hasn't been heard from since. Wouldn't be the first time.
Or you could have her take a bad step while climbing some mountain and fall to her death.

She opened Her eyes as the Telvanni closed the door to the chamber, her body was laying in pieces before her. She heard the click of a switch as the room began to decend into the lava pool. Babette stared back with her mouth open in a silent scream as the flames consumed Thier flesh.

Master Aryon looked into the eyes of the Night Mother and informed Her it was done.

Mephala laughed as She watched the strings of fate breaking on the loom. The betrayers would be forgotten and Her Morag-Tong would thrive.

EDIT Sheogorath laughed as the new thread passed through the Loom unheeded by the Webspinner, Azura was smiling inwardly as She watched the new Rose unfold.

Maybe the Frog brothers hunt her down