Citadel Altaerein is the abandoned former headquarters of the Hellknight Order of the Nail, located in eastern Isger, high in the northwestern Five Kings Mountains, close to the border with Druma. It overlooks the town of Breachill, whose residents call it Hellknight Hill and largely ignore it.
In 4638 AR, the Order of the Nail, impressed with the peace and efficiency of Breachill as well as its defensibility, chose a low-rising hill about ten miles northeast of town to build their headquarters. For decades, many citizens of the town took jobs in Citadel Altaerein, attending to the Hellknights' needs.
In 4682 AR, Queen Domina of Korvosa moved the Order of the Nail to Varisia, and they relocated from Citadel Altaerein to their new home Citadel Vraid. They kept a small handful of knights at Citadel Altaerein to keep an eye out for goblinoid uprisings to the southwest until 4711 AR, when their mounting missions in Varisia forced them to abandon their old headquarters entirely. All kinds of monsters have since moved into Citadel Altaerein.