A white dragon, antagonist of 01 Stellarium of the Vinteralf.
Thirty feet long with powerful wings, but slender enough for the tunnels. Ice-clear scales, tough as steel, and invisible when asleep. Breathes fire, likes her food cold.
In 5e Dungeons & Dragons
Challenge Rating 6: in order to fit through the tunnels, Jokun has to be no bigger than "large" size, meaning that she is a young white dragon (page 101 of the 5e Monster Manual). Jokun is too young to have any "legendary" or "lair" actions, which is a pity because they seem fun. But I guess this is what the whole problem is about: Jokun isn't quite tough enough to drive out foes like Thavir completely.
If I wanted to make this a little bit easier, Jokun may have been permanently blinded by Thavir's starsword Grugnir. If blinded, Jokun still has "blindsight 30 feet" even if she cannot see beyond that.