Problem At "Forgotten Treasures" Quest

Hi All

I am at Taranis island and I have a problem with this quest. There is a friendly shadow with the name of "Shax" located somewhere at the south beach of the island near shadow land. He gave me this quest and, as I didn't want him to accompany me, I told him to wait where he is. I found the package that he wanted where the shadow lord is located and then returned to him to complete the quest. The problem is, when I returned I couldn't find him. He was just disappeared. I checked my previous saves and saw that he disappeared some time later he gave the quest. In my logbook quest seems completed but I want to learn if this guy is somehow important for the later parts of the game. Because he was able to join me and it seem that he will be a part of my crew. I don't want to return my last previous save where he was still on the map, not disappeared because it is a very long way back. I also want to learn if it is somehow possible to respawn this character with a cheat to fix this glitch. His disappearence also disable the triggering of related quest "The Legacy".

Thanks