Archagruhn

 is a stronghold on the island of Solstheim, built by the sorcerer Velar Veleth in Fourth Era. By 4E 202, the stronghold is mostly destroyed by the ashfall, but the interior remained surprisingly intact.

Velar used Archagruhn as his hideout and base of operations after Veleth Ancestral Tomb, his previous base, was destroyed during Red Mountain's eruption. There, he researched the Ash Spawn as well as Trama Roots. The latter experiment resulted in overgrown roots severely damaging the fort due to Velar's mistake.

Tirvel Veleth, an undead battlemage, was held captive in the stronghold's Research Wing.

Archagruhn is discovered by Urjorahn, Yen and Clemence during the Dragon Crisis. The adventurers decide to besiege the fort and make it their hideout on the ash-covered island. Archagruhn is then refurbished and expanded, with added Smithy, Shrine and Bedchambers.

The fort itself is built like a traditional castle, with walls and smaller towers surrounding the half-collapsed great Watchtower, accessible via a staircase to the right of the gate. To the left is the entrance to the Cavern. The Watchtower goes beneath the ground, and serves as the passage to the collapsed chamber underneath. The ramp leads to Archagruhn Main Wing, which in turn houses the Library Wing and the Study Wing, complete with a Staff Enchanter, Arcane Enchanter and Alchemy Lab. A small chamber leads to Smithy and Bedchambers. Shrine to the Reclamations can be found in the cave near the Bedchambers. Treasury Wing can be found outside the Main one, and Research Wing is abandoned but accessible, inhabited by few Ash Spawn that broke free. Banners of House Telvanni adorn the Main Wing's entrance as well as the Watchtower.

Deep within Archagruhn exists the only static portal to Velar's realm of Foyada Padhome.

Trivia

 * Archagruhn is a mod for Skyrim, made long before the Padomaic Canon by Kool Khajiit. It also includes Foyada Padhome (labeled as Velar Ceysel in the mod), accessible through an altar in the fort. The mod can be found here. Requires Dawnguard and Dragonborn DLC.