Does not work at all with the latest patch. Neither before nor after the troll king cave - 0 effect. Looks like a bug if it is not intended.
That's weird. On my old save file (pre-companions patch), it didn't work in the sense that it gave 1 character 1 Tenebrium point and then was consumed (still only one use). After I started a new game, it only worked before I did the quest and it also worked for companions who weren't in my party.
