One hundred years is a long time and a lot can change, especially in the mountains.
A few additions/modifications to throw out there.
1) Make the time span shorter. I think you lose the legend of him if it takes 100s of years for him to come back. I think 10 would be plenty for the legend to sink a bit in age and the telling grow. Then when he reappears people will know who he is, trust him despite the fear, and maybe even keep him a secret.
2) Why keep him a secret? Because during the 10 years he is gone he continues to adventure in the other realm to find the necklace. During these times he finds treasure, hidden caves, mysterious beasts....etc. When he comes back, his knowledge of that which is not known is incredibly valuable. But finding him is pure luck so those that do don't want to lose him.
3) He continues to lead adventurers despite his fear because he has an additional legend on him that "The one who leads will be the one who is led. He who is led is the one that can be found again." Or something. So it is foretold that he will be found again, however he can only be found be leading others. Just has to find the right people that will bring him to his old life. So he continues to help any adventurers that ask in hope that they are the ones. Go to Comment
Ghoul with a motive, nicely done. Now, with such an overriding desire, how long will his humanity last before he starts to succumb to the darker impulses, when naked want begins to overpower the near incapacitating fear he has?
I think this is a very well-thought out item. It has uses beyond the typical cursed, or "magical" item as well. I can imagine a plethora of uses in my own games which could use this as a very interesting roleplay hook. Good job, Shadow!