Hello there.

Well, after searching far and wide, I wasn't able to find on the web any trigger or event that would make Bryling UNessential. Many have searched for this as well based on the sheer amount of folks looking for ways to get her special/ unique apparel, the fur trimmed cloak,boots and fine gauntlets.The only other way to go about obtaining anything similar from an apparel standpoint would be offing U. Stormcloak.

After starting my secod charachter I found out about the sometimes essential Bryling. I went back to my first character who was about 80hrs in and was able to off her easily in the Blue Palace. I heard rumors that the Dark Brotherhood quests were tied to her...so I tried with my second charachter. I wasn't able to kill her after that quest line. Some have hinted after the Thieves guild questline you are able to....alas is was not true.

So I reverted back to an old save and took three days to finish both questlines. She was able to be killed after these two quest lines were done! So to rock the fur trimmed cloak...finish the guild quests and you'll have them!

Forgot to mention my first charachter had these two questlines finished...so I was able to duplicate the results with my second character.

(*I realize I could edit my post above but my iPad won't let me scroll the text down in the edit window

Aside from using the console on PC to mark her unessential, supposedly you're able to kill her after becoming Guildmaster of the Thieves Guild. That's not the same thing as completing the Thieves Guild main quest line, as you also have to do all the "special" jobs for Delvin in Markarth, Solitude, Whiterun, and Windhelm.

I agree, but "Under New Management" is part of the main questline, where to complete you will need these side quests. I was implying by completing the questline, you need to become guild master, as evident in the skyrim wiki as well as UESP. (You truly are not finished with the questline until you reach Guildmaster.)

I also did try to kill her after guild master only (no DB), and was not able to. However after completing th DB questline WITH completing the TG quest line, only was I was able to kill her. I believe you have to have both completed as I tried each one individually from a previous save point in order to see what quest triggers the unnesrntial tag to be removed. Turns out you need both.