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 exit from Blackreach to this orb is the Great Lift at Mzinchaleft. The orb is visible upon entry. A number of Falmer and Falmer Servants will be hostile and one must thus fight their way through them. After doing so, use Unrelenting Force on the orb. Doing so will make the orb ring and echo throughout the caverns. This will summon Vulthuryol.