Thordak

Thordak, self-titled "the Cinder King" , was an ancient red dragon, even larger than most other ancient dragons. He was the leader of the Chroma Conclave. As an NPC, he was played by Matthew Mercer.

Background
Long ago, before maturing into an ancient dragon, Thordak lorded over half of the desert continent of Marquet for centuries. Eventually, he attacked Ank'Harel during its rise to prominence, but Thordak was gravely wounded and retreated. He was presumed to have died shortly thereafter from the wounds.

Thordak was not seen for centuries until about fifteen years before the events in. Rumors of a powerful drake, storming the beaches and seas, had been whispered for a long time while ships would be lost at sea. Thordak had survived, and secretly ruled the island of Viscaln in the south of the Lucidian Ocean. After generations, the dragon (now self-crowned as "the Cinder King") grew greedier than the island's limited resources could satisfy. He began razing the Mornset countryside, south of the Verdant Expanse and the Stormcrest Mountains.

Reports of a dragon—more massive than any on record—hit Tal'Dorei. At the time, Lady Allura wasn't a part of the Council of Tal'Dorei; she was a member of an adventuring party, much like Vox Machina. Allura's party travelled to investigate, but arrived to find this creature destroying everything surrounding the Gladepools, including Port U'daa and Byroden. (Vax'ildan and Vex'ahlia's mother was killed during Thordak's destruction of their hometown, Byroden.)  Gathering a small army of mercenaries, Allura's party hunted the creature back to its new den in the Stormcrest Mountains. They encountered resistance from a large number of lizardfolk, but eventually Allura's forces were overcome by the sheer might of the dragon himself.

Knowing they could not kill Thordak, Allura called upon the Arcana Pansophical, a circle of arcane practitioners that agreed to work together across the lands to keep them safe. They devised a trap that could, they believed, permanently seal Thordak within the Elemental Plane of Fire, preventing him from ever returning to Exandria. They bound Thordak's essence to the Fountains of Creation and thought he would stay there forever. To bind him, they crafted an artifact called a "soul anchor" from the salvaged heart crystal of a primordial fire titan. The heart crystal is an immense gem of pure crystallized ruby and elemental power and energy, fifteen to twenty feet in diameter. The soul anchor crafted from this heart crystal could not be removed from the Elemental Plane of Fire, and Thordak could not escape without the anchor. Although Allura's group succeeded in trapping Thordak, many lives were lost, including three of Allura's companions:  Sirus, Dohla, and Ghenn.

Vox Machina saw Thordak in the Elemental Plane of Fire while Keyleth progressed in her Aramente with the Fire Ashari. Thordak later escaped to the Material Plane after those events, helped by the ancient green dragon Raishan disguised as a young girl, an outcast whom the Fire Ashari adopted.

Thordak led the Chroma Conclave on an assault against Emon, Westruun, and (presumably) other parts of Tal'Dorei.

Lady Allura and Keyleth revealed through a scrying ritual that Thordak had returned to the Material Plane through Pyrah, devastating the area and killing druids of the Fire Ashari in the process.