I could not get Ch’thon Illusion. All other added and works correctly.
I can gain Illusion pack from LAncen, and I get message about adding new illusion, but it not appear at Illusionist window.
I repair game and restart Steam, delete transmutes save file, no result.
I also run custom game and spawn dlc_chthon_torso item by console, but it still not added to Illusionist as you can see here: