"Glinghant, the gardens of Annuminas -- once a place of great beauty, now overgrown and fallen into ruin. The stagnant of waters of the gardens have become corrupted by the fell spirits of Angmar, and evil Men do their bidding. . ."
This dungeon only opens when Unagh (Invader of Annuminas) and her three guards are killed, which opens the Dungeon Glinghant.
And that activates the quest npc, Harthadril.