The Fortress Twilight
The ancient edifice, now called the Sunless Citadel, was once the redoubt of the Vampire Lord Gulthias Sengir until the vampire was staked by a cursed spear to the Gulthias Tree by his forgemaster Hector.
In its current dereliction it has sunk into a ravine south of Oakhurst and north of Alsoor. The Palescale Kobolds and the Durbuluk goblins call this lair home while Belek ruled the dungeon from below, churning out monsters from the Gulthias Tree.
Our Heroes conscripted many of the inhabitants of the Citadel in their efforts to thwart Belek, but in the end the undead dragonpriest Miraak gained rulership of the Citadel instead. The Gulthias Tree, at least, has been destroyed.