The castle was built in 8 AO and was an attempt by the forces of Asmodeus to gain a foothold on Borendil after the Great Plague. The foothold was intended as a pre-emptive strike to wipe out the Humans, who had recently been seeded on Borendil by the Aesir.
The foothold was never attained, however, as the Aesir discovered the castle early enough and sealed the portal. They then terraformed a lake around the castle, so it would sit lonely on its own island, as it was still full of malevolant power.
The castle remained dormant until around 1828 AO, when the vampire Lord Somerset fled his defeat at the colony of Mendleheim and took up refuge in the castle. The vampire was mostly dormant within the castle, leaving only a few times to terrorize the countryside.