The Arch
The Arch is a large structure of white stone. It evidently existed in the area of what would become Diablotin prior to the arrival of the city's current human inhabitants, and was already an artifact of tremendous power. From evidence revealed in the Prehistoric Diablotin section of the first campaign, it was located at that time on top of a hill, which later became the Black Down. It seems that, at that time, people were sacrificed to the entity within it - whether as a form of veneration or placation is not clear. Dorea Lirette passed through the Arch and survived, but in the process the constellation representing the Rat disappeared from the sky, and it seems the god was somehow trapped within the Arch along with the original malevolent entity. The invaders moved the Arch to a new location, on what would later become the Imperial isle.
By the era of the first campaign, the Arch was located in a secret chamber beneath the palace, watched over by a guardian. It was employed as a means of testing candidates for the throne, who would each pass through it and emerge, one bearing some sign (never revealed) of having been chosen. Occasionally, unsuccessful candidates died in the testing process, but more often they were left alive but mentally unstable. Those who were deemed too unbalanced to return to normal society were kept locked away in the palace, and their ravings were sometimes recorded in the Codex Lunaticium.
The PCs in the original campaign discovered that beneath the Black Down was an inverted Arch, and the two had once been joined. In passing through the inverted Arch, they went back in time and experienced lives both as soldiers in the Quicksilver War, and as the heads of the original tribes who settled the region over two thousand years before. They were then returned to the present, seemingly unscathed.
At the conclusion of the campaign, Pasith Belden summoned an avatar of Ox, who carried the Arch back to its original location atop the Down, reuniting the two halves. This seems to have released the Rat god from its imprisonment, but also the other, more sinister entity. The Arch is currently a curiosity, and is venerated by some, but it no longer holds any magical power.
Old
The Arch is an ancient magical construct of unknown origin, which has existed since prior to the conquest of Saraknyal. It was formerly housed deep within the Imperial complex, where few people ever saw it. Candidates for the throne of the Empire, nominated by the Great Council, passed beneath it, and it was believed to select the one most fitting for the position. The unsuccessful candidates were often left mentally unstable or physically fragile, and some did not survive the process.
Ultimately the entity within the Arch proved malign in its intent, deliberately selecting rulers who would not live long in order to hasten its next 'feeding'.
The Arch was paralleled by a similar but inverted structure located beneath the Black Down. It was its move from its position atop the Down, and the binding of the Rat god within, which caused the fall of House Rat and many other subsequent problems. Its restoration to its proper place resolved at least some of those problems, but may have caused others. Only time will tell...