Trials of Chult
A large purple gem about the size of a small cantaloupe
Dendar the Night Serpent, eater of nightmares and prophesied to destroy the world. A creature from beyond the cosmos with power capable of battling gods. Dendar resides in Hades where she gorges herself on the nightmares of mortals of the Prime Material Plane. It is prophesied she will emerge from the Doors of Night somewhere in Chult when she is big enough to swallow the sun and freeze the world in cold and darkness.
A gem somehow related to her was discovered by Grave and Blackmoon in the the jungles of Chult. It was embedded in an idol that was capable of keeping a full grown primordial dinosaur at bay. It seems to resonate with negative energy and Grave was able to enter the area and secure the gem with no problem. Once the gem was removed from the shrine the protective field was removed but Grave and Blackmoon manage to escape the angry dino. Sleeping creatures near the gem do not rest well, gaining no benefit normal rest provides. If it has any other powers or uses it is not currently known.