The cellars of the ruined Covenant of Insulus Valens lead into an area of natural cavern. The portal to the caverns is sealed with an iron door inscribed with sigils and ensorcelled against passage from the other side. If the seals are breached it will be found that the tunnels lead into a complex of chambers, most of which are about five paces across.
The thirteenth hallow is fifteen paces across and has a pentagram inscribed on the floor, iron braziers at each of its points, and sconces on the walls. In this chamber nearly a century ago, the diabolist Inumbrus met his end against a Wizard’s March of Hoplites and Quaesitores while completing his vilest ritual. The walls of the chamber bleed a reddish-black, viscous ichor every All-Hallows’ Eve, the anniversary of Inumbrus’ demise. This contains Vim vis, but is tainted with the Infernal. Possession of it will draw the attention of demons, and deeds completed or spells cast using it will inevitably be turned to evil ends.
The deeds of Inumbrus were many and ghastly, and it is possible that some of his works survive to trouble the world to this day. Some of these works might be found hidden within the tumbled walls of the ruined covenant.