The Great Basilica in the Holy City of Ysithmir, the most sacred shrine of the Universal Church of Gilded Dagon, was built on a sacred precinct whose history stretches back to time immemorial. For most of its history, though, the site housed a modest temple of a faith now lost; the modern structure postdates the Convection Engine, without which, indeed, it could not have been built. Although its flying buttresses have been condemned by one noted architectural critic as "a bad pun", the building is more commonly noted for its three beautiful mosaics depicting the lives of the three Patriarchs. After walking through the entry hall and viewing the mosaics, a worshipper would enter the Basilica proper and approach the Great Altar. It is entirely unclear how the Church of Dagon obtained enough Cloudstone to levitate the entire platform, but the effect is said to have been quite impressive. For the Dagon faithful, of course, the floating altar represented Man's ascension into the heavens and ultimately into the Divine embrace.

Some have claimed that the Basilica's design reflects the Aetherists' progressive values, with the floating altar and flying buttresses representing Aethereal aspiration, but this strikes the present scholar as a reach. Surely the altar represents an aspiration to the Air and no higher, and while the Aetherist tendencies of certain high-ranking Dagon clerics is well-documented, there is no evidence that their influence extended to the Holy City itself. More likely, the rationalist aesthetic of the Basilica is a reflection of Nagadaan, indicative of the Dagon Church's receptiveness to the ideas of other faiths.

By far the most notable event in the Basilica's long history is the still-unexplained disappearance of the entire building the midst of the Little War, and its subsequent reappearance exactly one lunar year later. In the famous words of High Beneficent Toth, "First it was there, and then it wasn't. And then there it was again." Contemporary explanations for what became known simply as the Disappearance ranged from Aetherist plots to divine intervention, from a new weapon to a new manifestation of the Aberration. While this last seems the most likely--indeed, the only plausible--explanation, official blame for the Disappearance came to rest with the Dagon Church's hierarchy, severely disrupting their influence and destabilizing the entire region. Indeed, it seems fair to say that had the Basilica not disappeared, the Sandalphon Affair would never have occurred, or at least would have occurred very differently.

—Iohannes Edgardus Quobertius