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 you summon Vulthuryol but do 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 you need to return to Blackreach, so long as you spot him first and approach from the rear. Note: Vulthuryol can only be summoned once.
If you summon Vulthuryol but do 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 you need to return to Blackreach, so long as you spot him first and approach from the rear. Note: Vulthuryol can only be summoned once.