There is no such thing as Tenebrium ability anywhere.
It should be listed in the Abilities tab in the inventory window, under Weapons, between Single-handed and Two-handed, once it is unlocked. It is this ability that is required to add tenebrium damage to a weapon (level 3).
If you check that and still have a problem, please email supportdos@larian.com, with a brief description (or link to your post) and the report.zip file (or a Dropbox or Google drive link, etc) generated by the D:OS EE Analysis Tool (for Windows). Extract the zip file somewhere and run the AnalysisTool.exe. When that loads, browse to the install folder (if it is not automatically detected), optionally run the tests first, and then generate a report (it will be saved in the same location as the Analysis Tool).
The report will contain system and game information, as well as your saved games. To reduce the file size, you can delete most of the saves from the zip; leave one before completing Brandon's quest, if available, and your latest (if characters don't have the Tenebrium ability listed, or can not craft at level 3).
This is my fourth playthrough of the EE, and there has not been any Tenebrium ability so far. This has been discussed in this forum several times since release, but the ability has worked, without being visible. Now it doesn't. It's not just me.
I'll just give this game a rest for a few months, until you guys sort things out. There are just too many issues piling up, and I don't want to spoil the experience and raise my blood pressure. I really hope it's fixable, because I loved the original game, but the EE for PC has been nothing but a long slog through frustration for me. Sorry.
