The fanfiction being referenced is Decadent Habits, by Darkenning. It's actually a lot better than the following would have you believe.
Decadence and Folly
How the Darkenning sect's unique interpretations of certain deities reveals the magnitude of their depravity
The infamous Darkenning sect, more commonly known by their epithet "the Decadents," was the great shame of the Akamatsu. They ignored the useful symbolism of the pantheon in favor of wild and lurid tales of wanton debauchery between the inexplicably related Ala Albans, which many used to justify incestuous relationships. It is generally agreed that this bizarre heresy had its roots in cross-pollination with another pantheon or pantheons. Classical mythology, Egyptian mythology, and the incest between Arthur and Morgase are all commonly cited influences. Regardless of the source, the ideas were swiftly and enthusiastically adopted by the lowest sort of people. Nevertheless, a true scholar may produce a silk purse even from such a collection of sows' ears as the Decadents.
Evangeline A. K. McDowell, vampire and slaver, is widely understood as a symbol of dangerous appetites, historical barbarism, and the abuses of power (such as the power a lustful parent might have over an innocent child). It seems even the Decadents may have on occasion grasped the symbolism of their deities, as they reduce the anthropomorphized warning against their most beloved forms of depravity into the infantilized "Kitty." Where Evangeline was meant to disturb worshipers by combining the appearance of a child with a lustful demeanor, Kitty is a child in truth without losing her carnal purview. That the Decadents would thus render harmless the great bogeyman of Ala Alba, and then go so far to ohteepee her to Negi, shows a contemptuous disregard of the consequences of their actions that would be shocking among any less depraved sect.
While the Decadents accomplished the impressive feat of making the tales of Evangeline yet more disquieting, they have made those of Haruna more wholesome. Give Haruna's role as a patron of chaos and certain forms of erotica, one would expect the Decadents to embrace her with open arms. Instead, they seem to have subscribed to Hero Theory, and judged the supposedly historical Haruna as overly prudish, rather than understanding her as a representation of their own impulses. Why the more open Haruna was held to this "historical" standard where her fellows were freely warped is one of history's great mysteries. How may the well-adjusted fathom the minds of degenerates?
Another relatively unscathed figure in Decadent mythology is Makie. Her retention of her innocence has been cited by certain scholars as evidence that even the Decadents had depths to which they would not descend. These scholars are naïve. Was not Chachamaru, retaining her aspect as the gentle protector of cats, sucked into the incestuous maelstrom? One must not overlook the fact that, in addition to being an innocent, Makie was also described as a fool. Stories of her being crushed by her misfortunes and inadequacies were popular among children at the time. While we may see Makie's innocence as positive, the Decadents would have viewed it as worthy of scorn, a fitting fate for a failure.
There is one final abnormality worthy of note among the Decadents' legends. They were the only known sect to believe in an ohteepee between Satsuki and Zazie. A connection drawn between welcome and peace on the one hand and enigma and shadow on the other would seem a matter of great interest. However, given the nature of the Decadents, it is far more likely that some priest or other merely found the coupling of those particular deities erotic, with no thought given to deeper meanings. The mind reels and the bile rises at the thought of such a mindset.
Now the Darkenning sect is as fully understood as it need be. A thorough investigation of their theology shows that the common contempt in which they were held was, if anything, insufficient. The Decadents' theology and practices were unrelentingly vile, and they should be ever remembered as a warning of how low humanity may sink if its vices are not guarded against.