The Darkhold is the Last Holy Site of the goddess of Perfidy. Back in the Degenerate Era, someone built a massive fortress on the location, and was apparently exterminated before anyone started recording history again in the Anyube River region. The nations of Noc and Tyh spend a lot of time and energy fighting over the place.
Not to say either side has managed to do anything with the site. The gates don't close, the outer bailey isn't inherently inimical to mortal life, there's even a well. Holding the fortress is challenging, given the gate situation and the well only being up to supporting a company of troops at best. The gates to the second curtain wall only open for an hour around noon, and there are hungry things lurking in the shadows beyond them.
The situation gets progressively worse and terrifying as one works their way through the five layers of walls to the Cathedral of Gidah. At present there are no reputable reports of anyone making to the Cathedral and getting out again alive.