Gelidon

 is an ancient white dragon. As an NPC, is played by Matthew Mercer.

Appearance
Gelidon is "of hoarfrost color"; in other words, a white dragon.

In the Vellum Steeple Archives, Beau read a book that spoke of a cursed location to the far eastnortheast of Uthodurn, known as Mythburrow, about 350 miles away. It talked of an ancient dragon of hoarfrost color and quaking roar that was thought to possibly nest there, but unseen in nearly a century. The dragon’s name is Gelidon, the Nightmare in Ivory.

The Mighty Nein and their aasimar druid ally Reani attempted to scope out Mythburrow, Gelidon's supposed lair, with hopes of turning the mythril bar in their possession into iceflex via a white dragon's breath. In doing so they killed the present members of a group of yeti living inside, and discovered cave drawing-like depictions of the yeti delivering gifts to a large, winged lizard. Knowing that white dragons prefer freezing their prey before eating, the party hatched a plan to bait Gelidon out with an offering similar to those of the yeti, but hid the mythril bar nearby in hopes it would be blasted by proximity from the dragon's icy breath. Once everything was in place, the massive form of the ancient white dragon Gelidon burst out of his lair to inspect the offering, and killed it with two subsequent frozen breath attacks. Everything seemed to be going smoothly for the Nein until Gelidon tore its frozen prize out of the ground and flew back to the lair with the mythril bar still attached to the underside. With no other options and noticing that the bar had been turned into iceflex before being carried off, Nott, Jester, Beau, and Reani (in giant owl form) snuck into its lair.

Quotations

 * “Where do you think you're going?" -- deeply inhales through his nostrils, all the while pacing back and forth. "I know your scent now,” in a singingly sinister tone.