|Romaji||'Shikai' no Gehena|
|Light Novel Debut|
'Hell' Gehenna ('死界'のゲヘナ, 'Shikai' no Gehena) is the First True Dragon, the first of the calamities that have threatened the existence of Earth as a whole. It was presumably eliminated by Gaia in the distant past through unknown means.
According to Tear, Gehenna is an umbrella term for external foes that had arrived to the world, meaning it doesn't have a specific appearance.
Gehenna arrived on Earth at an unspecified point in time in the distant past, possessing the capability to wipe out all life on the planet. In response to its arrival, the Earth developed its own will, known as Gaia, the first Counterdragon, which presumably killed Gehenna through the use of its Authority, Code Eins, and would proceed to create the other Counterdragons to counter the subsequent calamities that came after Gehenna.
- Gehenna's name is derived from one of the alternate names for Hell in Christian Mythology.