Succeeding Celestius VIII and both succeeding and preceding Pope Sennis as pontiff of the Universal Church of Gilded Dagon, Celestius IX is best known for presiding over a marked downturn in Dagonic influence that would provoke the transformation of the Church under his successors.

Like many popes arising from the Vitalicant, Celestius maintained a striking focus on the purification of the faith. Celestius, however, was unique in his belief that there existed an organized conspiracy to corrupt the Church from within. From year to year Celestius's suspicions shifted dramatically, settling first upon the growing influence of technophilic cults such as Nagadaan and promoting a short-lived alliance with the Naturalists, then upon the Naturalists themselves, prompting a denunciation of aether-safe flight in his famous encyclical Earth for the Raping. After battling the ruling houses over his denunciation of the Straylight Reforms, Celestius settled for excommunicating Dwight Lansford, the entirety of the Pulotu people, and each of the remaining Septarchs before launching the Gilded Horde on an invasion of his own papal states in order to preserve the sanctity and security of God's people.

With its resources overstretched and its prestige at a nadir, the Church sought new leadership, forcing Celestius from the papal throne and replacing him with the reformist Pope Sennis. Celestius might have lived out the rest of his days in obscurity were it not for the sudden disappearance of the Basilica of Ysithmir, and Celestius's successor along with it, forcing Celestius's temporary reinstatement as pontiff under an obscure quirk of canon law.

Celestius devoted his short-lived second papacy to undoing Sennis' radical reforms, but his attempts were futile, undermined as they were by Sennis' well-entrenched allies. This deepened Celestius' conviction that a grand conspiracy was at work, claiming that a host of "invisible invaders" had infested the Church and besieged the Holy City itself. His weekly encyclicals became increasingly strident, until they consisted solely of lists of the newly-excommunicated.

It is Sennis' reappearance one year later, along with that of the basilica, which appears to have broken Celestius completely. Refusing to step down, Celestius declared Sennis to be "in league with dark and hidden powers set upon our corruption" and ordered the younger pope hung and burned for the good of mankind. Celestius' orders were ignored, but he was granted a formal disadjudication hearing in which to contest Sennis' papacy. It was there that Celestius revealed that he had seen, over the last several years, "dark phantoms appearing from the air in the form of angels and of demons" in the area of Ysithmir; that these invisible agents had permeated the Church and stolen both Sennis and the basilica; and that Sennis had not encountered the physical manifestation of Gilded Dagon as he had claimed, but a monstrous evil to which he had succumbed.

While Celestius' testimony confirmed him as an incurable lunatic in the eyes of his contemporaries, it has provoked a more favorable reassessment in recent years in light of the Sandalphon Affair. Indeed, many of Celestius' writings read like a description of early contact with the Zone of High Matter, and the circumstantial evidence would be compelling were it not for Celestius' known addiction to the Red Laquor, an indulgence which would have certainly decayed his mind by the time of his second reign.

Celestius spent his remaining years in a protracted attempt to decanonize the full tier of established Dagonic saints and paragods on the basis that "all are poisoned by madness, demonism, and the foulest taint of bestiality." His efforts largely came to naught, and only succeeded in temporarily delaying the elevation of High Beneficent Toth to the status of Living Saint.

Celestius IX is the only Dagonic pope to pre-emptively refuse sainthood, and remains the only pontiff not canonized by the hierarchy today.

—Rudgaard Vanderplast