Arx Corinium was an Imperial Fort built within the province of Black Marsh sometime, most likely, during the late First Era or early Second Era. The Arx Corinium was the Imperial Legion's attempt to create a fortress within one of Black Marsh's swamps as to gain a foothold in that region. Unfortunately, the construction of the fort was severely hindered by environmental means. The fort's construction team lost half a dozen workers each month due to the varying dangerous factors of Black Marsh, and the fort's initial foundation sank below the swamp.
The heat, wildlife, and diseases of Black Marsh continued to take a toll on the construction, the eastern wall section was said to have been razed a dozen times by a Wamasu known as "Ganakton the Tempest." Despite the terrible conditions, the fort was successfully constructed, though the extreme amount of manpower and resources needed to make it almost made the fort not worth building.
- Ancient Lurcher (Lurcher) (Swampbinder)
- Fanged Menace (Giant Snake)
- Ganakton the Tempest (Wamasu)
- Matron Ixniaa (Lamia Matriarch) (Matron's Mark)
- Sellistrix the Lamia Queen (Lamia)
- Sliklenia the Songstress (Lamia Matriarch)
- Sister of Floods (Nereid)
- Sister of Rain (Nereid)
- Sister of Waves (Nereid)
- "Arx Corinium Vanquisher" achievement is earned by defeating Ganakton the Tempest, Sellistrix the Lamia Queen, and Sliklenia the Songstress.