Soulstone

In 18 ABY, Darth Odium created an artifact out of a Force crystal shaped as an octahedron, and imbued it with Sith magic that was able to ensnare a Force ghost that was within its radius. He passionately called this Force-imbued crystal the Soulstone. The young Sith Lord first used the crystal to cage Darth Nefarious's spirit, rendering him powerless and successfully foiling his attempt to possess Odium's body. While Darth Odium crafted the Soulstone, and knew of its exact purposes, it remained unclear if the artifact had any further purpose than this.