Solomon talked about the Star of David (a.k.a. hexagram).He said that although many jews thought it was a holy symbol, in reality it was a really evil one. Specifically, he wrote in the key of Solomon that the shape of the star of david is actually made in the shape of a demon's footprint.
Now, about your problem, I suggest that you get rid of that pendant.Or at least, attempt to purify it.It seems that this pendant may be charged with a negative spirit, or it could also summon energy autonomously(including spirits of any sort) due to its shape, but I am not really sure about that last one.
Try doing some banishing spells too.
Wish you good luck.