Vulthuryol

Vulthuryol is a hidden Dragon summoned in the underground Dwarven city of Blackreach.

Summoning Vulthuryol
To find Vulthuryol, access the center building area in Blackreach, where there is a large yellow orb above the Blackreach Debate Hall and the Hall of Rumination. The closest entrace/exit in Blackreach to this orb is the Great Lift at Mzinchaleft. The orb is visible upon entry. Make your way through the Falmer and Falmer Servants and use Unrelenting Force on the orb. It is possible to do it from in front of the Debate Hall entrance if you want to go through. Doing so will make the orb ring, and echo throughout the caverns. This will call Vulthuryol to attack you. A simple yet effective method to defeat Vulthuryol is to use Dragonrend to make him land outside the walls and use ranged attacks on him while constantly bombarding him with Dragonrend.

If the Dragonborn summons Vulthuryol but does not engage him before leaving Blackreach, he may eventually take up a stationary position on the ground to await developments. This makes it relatively easy to ambush him on a later trip if one needs to return to Blackreach, so long as one spots him first and approaches from the rear. Note: Vulthuryol can only be summoned once.

Trivia

 * Vulthuryol's name translates to "Dark Overlord Fire".
 * It can be assumed Vulthuryol was not killed by the Blades and is a Dragon who survived the Dragon Wars, since he is in a Dwemer ruin deep below the ground. It can also be assumed that nobody on the surface knew about his existence.
 * Unlike other dragons, Vulthuryol will not fly while fighting the Dragonborn, but will crawl on the ground, using Fire Breath against the player.
 * Vulthuryol drops standard loot for a dragon.
 * It is possible that Vulthuryol will not see the Dragonborn when he spawns. If this happens, he will land and crawl around. Using shrouded gloves and the assassin's blade perk along with a powerful dagger will make him very easy to kill in just a few hits.
 * It is unknown how Vulthuryol became trapped in Blackreach.

Bugs
 * 1) If shot by both a deadric bow and arrow Vuthuryol will crash and not get up
 * 2) If wearing ebany mail and fighting him the dragonborn will not absorb his soal

Images
Vulthuryol in Blackreach