I've got a workaround that is a serious hack. Only for advanced users. It involves hex editing "Globals.lsb", which could very well destroy your save and possibly render any progress impossible. I just successfully changed my save so that I could talk to Brandon again, get the quest, complete it, and then get the Tenebrium skills as expected. Who knows if my save will be bugged in some way I can't foresee quite yet.
If you're interested in trying it out, I'd suggest getting an open source hex editor like "Frhed". Navigate to your save folder (eg. <My Documents>\Larian Studios\Divinity Original Sin\PlayerProfiles\<PROFILE>\Savegames\QuickSave_4\) and load up Globals.lsb in your hex editor. PLEASE BACK UP YOUR SAVE BEFORE YOU MAKE CHANGES.
In the editor you will probably see a bunch of hexadecimal code on the left, and some text on the right which is a text representation of that code. You'll notice some keywords that look familiar. Do a "Find" for "LUC_Brandon_Bothered". This will get you to the section of the save that records quest and dialogue progress. We can modify it so that the progress is corrupted and reset to default.
After you search for "LUC_Brandon_Bothered" in "Globals.lsb" of your save, you can begin editing the code on the left. Look for the following patterns:
"4c 55 43 5f 42 72 61 6e 64 6f 6e 5f 42 6f 74 68 65 72 65 64 00 0c 00 00 00 04 00 00 00 XX 1d 00 00 0a" where "XX" may be something random (it was 5d in my save)
and
"4c 55 43 5f 42 72 61 6e 64 6f 6e 5f 50 6c 61 6e 00 0c 00 00 00 04 00 00 00 XX 1d 00 00 0a" where "XX" may be something random (it was 46 in mine)
Whatever you have where the "XX" is in those lines, change it to "00". This should clear the quest progress and let you talk to Brandon as if he were a fresh NPC again. Don't change the other "LUC_Brandon" entries, or you may automatically complete the quest with unpredictable results. The first part of those two lines are the labels "LUC_Brandon_Bothered" and "LUC_Brandon_Plan", and the second half contains code for the quest progress. By changing the segment before "1d" in those lines, we reset the status.
I can't guarantee that this will work for you, but it allowed me to get and complete the Troll King quest. It's not for the faint of heart.
