Context First: The BBEG my players are up against was originally a mortal who became the herald of a dark deity, the god of the end times. The first time he attempted to destroy the world (approximately 10,000 years ago), he was executed. In the moments before his death he used unknown magic to forever bind his soul to the material plane, making it so that when he died he became a roaming spirit so that he may some day return. Long story short, he has returned several times and attempted to complete his god’s work to destroy the pillars that hold together the world, getting closer every time, but never fully succeeding. The last time this happened (about 1500 years ago), he was killed by one of the archmages of the world’s largest school of magic and his soul was placed into some sort of vessel, which was stored away and eventually lost. About 10 years ago, this vessel was found and he was able to manipulate the person that found it into releasing him.

My party is eventually going to find the place that he was released from, and they are going tom come across the shattered remains of this soul-vessel, but I am drawing a blank on what this vessel should be. I don’t want it to be just any old magic item, it want it to be some that when described to the party it conveys how serious it was that this person was trapped. What sort of thing would you use to contain a quazi-eldritch soul with the knowledge and the drive to literally knock down the foundation of reality?