[One-armed Prince] was a rather ancient evil god, one of the earliest gods currently traceable to being worshipped by mortals in the Material World, aside from the Ancient Gods.
This god wielded the power of Flesh Aberration, and most of his followers were disabled. Ordinary people who wished to truly enter the inner ranks of his cult had to cripple their own bodies.
The followers of this evil god believed that Flesh Aberration was the only correct path for mortal beings to continue evolving. Thus, they devoted themselves to all manner of horrifying human transformations, flesh research, and experiments.
The infamy of the [Aberration Cult] had endured from the Mythical Epoch to the Sixth Epoch. Records of their evil deeds existed in every era. Even in the Third Epoch, when Demons ran rampant, they were among the few groups that ignored the survival of civilization and persisted in infighting within it.
Shade knew so much about this cult because, in the 1840s before he appeared, the [Aberration Cult] had attacked [Snake Heart Hospital] in the southeast of the Old Continent. Only when Sister Devlin personally intervened did she, together with the Ring Arcanists of the local Church, manage to bring back most of the abducted children.
This had also been one of the few times Sister Devlin had directly interacted with the Orthodox Church before the Outlander arrived.
The title "One-armed Prince" was related to the Ancient God who had created him, the [Twilight Creator]. The Old Gods connected to that Ancient God all possessed alternate titles such as "Princess" and "Prince." This seemed to be because the gods of the "Creation" lineage had attempted to strengthen their respective connections through such titles.
For example, one of the three evil gods worshipped by the Doppelgangers in the [House of False