after 117 mistmoore missions I finally got this npc. Was in Dreary Grotto. Dropped the Girdle of Bloodstained Stones. Maybe the Matron only shows up on Sundays ( today is Sunday 9/23/2018). In game it is Wednesday, 3/25 though.
Edit: She popped up again today on Friday (9/28/2018) in the Chambers of Eternal Affliction - so not Sunday only. It is Wednesday in game again though ( 7/11). Mission number 262. and I still have not seen Trueborn element Wielder.
Edited, Sep 28th 2018 8:37am by Shuraz