I just remade my main file for the third time (the first time she was too ugly and then after I remade her and got to 54 I noticed she had a weird nose) and I finished the Thieves Guild questline. I never gave it much thought before, but
Karliah tells you that you will need to defend the Twilight Sepulcher if it ever becomes under attack again. Since Google searches won't give me any relevant quests (or forum threads) and I've certainly never come across anything in-game, it seems like this is a setup for DLC. Thoughts?
It was just a handwave to give you a good enough reason to not have any more duties after the questline is over. "You sold your soul to Nocturnal, you are the protector of the Twilight Sepulcher now. You've done your duty, you are free to go for now and will be called back when needed."
Need voice to think in? How 'bout ZOIDBERG why not?
That's why I think this is hinting at a DLC. I guess it would be annoying if you would randomly be called back to the Sepulcher, at least as long as you could ignore it if you wanted without any consequences. It just seems like it wouldn't have made sense to include that warning if it wasn't ever going to be utilized. If they never planned on having you actually have to come back there, they should have just said that after you die your soul will be bound there in order to protect it.