Skysunder

Skysunder, Herald of the Rimelord was a white dragon and scout in service to Errevon the Rimelord who had been sealed within the Frostfell, the Elemental Plane of Ice. She was one of Vorugal's children. As an NPC, Skysunder was played by Matthew Mercer.

Appearance
When Vox Machina first saw Skysunder, she had the appearance of a pale, nude woman with white hair and a frail physique. It was only after they chased her through a portal that she revealed her true form to be that of a white dragon.

Background
At some time prior to or concurrent with the Scattered War, Skysunder came into the service of the ice titan Errevon. She acted as a scout in order to find the weakest points between the planes. It was with her help that, two years into the reign of Sovereign Zan Tal'Dorei's reign of her new kingdom, Errevon opened a portal in the forest now known as the Frostweald and plunged Tal'Dorei into an eternal winter known as the Icelost Years. At some point, Skysunder was defeated and sealed within the Frostfell.

Vox Machina Origins
Volume three In 809 PD, during the Winter's Crest Festival in Westruun, a mysterious blue crystal was delivered to the town. The crystal suddenly detonated, freezing everyone except Vox Machina, who were protected by Abjurist Noja. In the crystal's place stood Skysunder, in the form of a pale, nude woman. She opened a portal and leaped through, closely followed by Vox Machina. On the other side was an icy, frozen world, the Frostfell. Skysunder revealed her true form as a white dragon, although she was weakened by her premature rebirth. The party managed to defeat her, with Pike Trickfoot landing the killing blow, and safely returned to Westruun, where the citizens were released from their icy prison.

Vox Machina did not learn the name of this dragon until Allura Vysoren told them that they had defeated one of Vorugal's children, Skysunder, two years before the Chroma Conclave attacked Emon.