Go to a place called Sacred Stone, the portal to this place is in the trap door in Loic's chappel, you can either complete the immaculate trial to get the key for the trap door or you can just break the door.
In Sacred Stone there is a lady immaculate called "Attenberah" or something. She has a ice and fire elemental in front of her room. Don't go in and talk to her if you not high enough level as she is very likely to fight you and then the rest of the town will fight you as well. If you have invisible skill or potion, use it and go in. There is a book called "How to handle Tenebrium" on one of the shelves, just steal it and come out of the room. Once you have this book give it to each of your characters and make them read it to get the Tenebrium skill.
