Undead Ancient Dragon

The Undead Ancient Bone Dragon(アンデッドエンシェントボーンドラゴン) was one of the monsters in "Hachinan tte, Sore wa Nai Deshou! Wiki "

Appearance
The Undead dragon is the huge skeleton of a deceased dragon with red eyes and a large red energy crystal in its chest cavity

Synopsis
While Wendelin von Benno Baumeister and his party where traveling on the magic airship heading to the Capital for his brother Erich Baumeister wedding the dragon was going to attack the airship. While Brantark Ringstat was protecting the airship Wend battled the dragon by using Holy Magic. After defeating the dragon Wend was made a Baron, given the twin dragon award and given 170,000 gold coins for the dragon and its energy crystal. Defeating this dragon is what started Wend's journey to becoming the most powerful mage and nobles in the Helmut Kingdom

Classification
Calamity Class or Legendary class monster depending on translation.