Soul anchor

A soul anchor is an object that provides an arcane means of binding a creature's soul to a specific plane.

Appearance
Thordak's soul anchor, the only known soul anchor, was a smooth, jagged-edge ruby that was embedded in his chest and appeared to pulse.

Properties
The soul anchor served to bind Thordak's soul to the Elemental Plane of Fire: it could not leave the plane, and Thordak could not leave without the soul anchor. It also had the side effects of increasing his size considerably; mutating him such that he could reproduce asexually; and damaging his sanity.

History
Approximately 15 years prior to the Chroma Crisis Allura Vysoren and her adventuring party, upon realizing that defeating Thordak was beyond their capabilities, called upon the Arcana Pansophical to devise a means of trapping Thordak in a place he could not harm Exandria. They successfully sealed him in the fire plane, using a crystal crafted from the heart of a fire primordial.

Some time later, Raishan approached Thordak, still trapped in the plane of fire. She attempted to destroy the soul anchor, but found it indestructible by the means available to her. She, allied with fire giants, was ultimately able to, after infiltrating the Fire Ashari tribe at Pyrah such that she had easy access to rifts to the plane of fire, find a way to capture some of the essence of the plane within the gem. This permitted Thordak to leave with it and return to the Material Plane.

Gilmore noticed the gem during the initial Chroma Conclave attack on Emon, and Allura independently told Vox Machina about it when recounting her history with Thordak. While Allura at the time believed it to be indestructible, Vex'ahlia was able to destroy it using Fenthras, a Vestige of Divergence, when Vox Machina fought Thordak during the Siege of Emon.

Vox Machina later discovered, upon following Raishan to the Island of Viscan, that the soul anchor had permitted Thordak to spawn primordial dragons.