I would like to know about this too.
I never got the quest from Brandon in the first place and I feel like Larian handled this quest very poorly. When I entered Silverglen I started talking to everyone like I learned was a good idea from Cyseal and the one common thread I got was that tenebrium was bad and it gives rot which will kill you very quickly and then I spoke to some guy (Brandon) who wanted me to handle some of this tenebrium stuff and of course I declined. Little did I know that rot was absolutely pathetic and that tenebrium weapons were not only very powerful but apparently necessary at some point.
