The Gineden pendant is a disposable in Metal Shadow Prelude. It is a pendant which Vantrekke Yuba used to seal the spirit of Gineden. He used it to draw energy to complete a task in the collapsing Agency HQ.
Since Van (who is really Lad Wydnie) died, the pendant's whereabouts have become a mystery. It may have fallen into the Three Queens' hands because they could only use Gineden's powers to escape the Cloud Wall.