Did you try switching which main character talked to Brandon (and had the blood stone cage)?
You can find a tenebrium skill book in Sacred Stone.
Please email supportdos@larian.com, with a description of the problem and the report.zip file generated by the
D:OS support tool (for the PC version). The report will contain system and game information, any crash dumps, and your saved games. If file size is an issue, you can delete older saves (or ones in other profiles, etc) from the zip.