In the Dark Brotherhood's Sanctuary, near where Babette usually sits, there's a spider. This spider is unique in that it is not aggressive, and does not even appear labeled or named when you approach it.
If you kill it, you will find that its corpse is named Lis and it carries the usual spider loot of Frostbite Venom and the occasional miscellaneous items. It also has a Petty soul, which is fairly normal for a spider of its size. And then, you'll quickly find all the members of the Dark Brotherhood (except Babette, as she is technically a child) are now aggro. This can occur even if you were hidden when you made the kill.
However, I've yet to find any in-game source of a back-story for Lis. Can anyone shed some light on this?