Yeah, i was thinking about setting it on the editor because i tested it and nobody else gets the Tenebrium skill (I have a separate character for bartering and neither he nor jahan and madora get the skill if i tell them to leave and then get them back) after that quest, which leads me to believe it's a one-time event.

The only other source for the tenebrium skill would be the book, and it doesn't solve things for the 4th character.