Black Dragon

The Black Dragon is an enormous dragon that is the boss of Tonnesgrave. It ends up under the Geniuses' control during Log Horizon 9: Go East, Kanami! and poses a serious threat to the Adventurers in Aorsoi.

Overview
Originally, in the vast land of Aorsoi, this dragon wasn’t a particularly enormous or difficult enemy. It was a level 85 full-raid monster that could be defeated by 24 Adventurers of equal level.

It was normally found guarding the ruined base of the iron pillars that was the defying feature of the location.

Go East! arc
The dragon was found asleep by Papus and Rasphuia. Rasphuia took it under her control and used her ability, which allowed her to transfer the names and souls of minions under her control, to turn the Black Dragon nigh-invulnerable. Any direct attacks that would have harmed the Black Dragon had their damage transferred to other minions.

The two Geniuses used the Black Dragon to destroy the "Iron Pillars" (rocket launchpad) at Tonnesgrave, which they referred to as the "Corridor to the Heavens." The Lelang Wolf Cavalry attempted to stop them twice, but were wiped out both times. Upon hearing this, Kanami's Party, sans Leonardo who had reservations about fighting a hopeless cause and KR who stayed back to accompany Leonardo, ventured down to Tonnesgrave to confront these "undying monsters" themselves.

With the help of KR's pacted Garnet Dragon, Ga-tan, KR and Leonardo flew to Tonnesgrave in order to handle the dragon themselves. By the time they got there, they found that it had already destroyed the Iron Pillars. At this time, they noticed the two Geniuses riding on top of it.

Leonardo tackled both of the Geniuses from the dragon's back and confronted Rasphuia personally, while Kanami, Coppelia, and Elias Hackblade fought Papus. Meanwhile, KR drew the dragon away from the area and engaged it in aerial combat with Ga-tan. It proved to be too difficult, so KR decided to dissolve Ga-Tan's summoning contract so her full abilities would be able to unlocked.

With her full capabilities and KR's support, they managed to defeat it. However, KR took too much damage during the fight, causing him to faint and die from fall damage. He later revived back in Yamato.

Abilities
The Black Dragon's massive body gave it a large amount of stamina and HP, but it wasn't as maneuverable as the smaller and slimmer Garnet Dragon. It could fly and use its tail to deal out deadly swing attacks. Its Negative Thunder Breath skill also dealt lots of damage.

While under Rasphuia's control, it was rendered nearly invulnerable thanks to her ability to transfer souls and names, thus most of the damage it took was transferred to other monsters than itself.

Skills

 * Negative Thunder Breath
 * Lightning is gathered in a black, warped space, then shot from the Black Dragon's mouth.

Trivia

 * The anime skipped over the part where it is taken over by Rasphuia, as well as the following fight with KR and Ga-tan.