Quote (Lublar @ Apr 5 2012 02:39am)
I received my blade from the thieves guild quest at like level 40 something, don't remember exactly when, and I was able to upgrade it to legendary with no problem.
So, I'm not sure, I didn't have any problems though, and I had the highest version of nightingale equipment.
highest version of it is at lvl 46+ it should do 25health/25stamina