Nidhogg

The Black Dragon (黒い龍) who goes by the name "Nidhogg Dragon" was a ruler hailing from the 12th Realm who was sealed in one of the old abandoned bases of the Cult of Diablos near the academy town of Laugus until it was awakened by Eta by mistake, whereupon this Dragon was slain by Shadow.

Personality
Unlike the other Dragons, from the other world, which resemble the Black Dragon, this one, for its part, is wild, it is not intelligent, and it is carried away by its instincts like any animal or wild creature.

Powers & Abilities
The Black Dragon was a magical being of great power, which was said to be able to sweep an entire country, in less than a day. Said Dragon was so powerful that not even 2 of the 7 Shadows of Shadow Garden, Zeta and Eta, in full capacity and growth, both in magical power, and in the rest, could defeat it. It was not until Shadow happened to pass by there, when feel the awakened magical power of the Black Dragon, and at the end, Shadow killed him with a "I Am Atomic".