Octopusism

A religion whose sole goal seems to be the destruction of the world by or on the year 1000 AD, either through a flood of water or a scorching of the world with fire (and it's looking more and more like fire).

Tailed People are the biggest worshippers of this religion, but there are others who, if they don't subscribe to the religion, they're at least allied with the religion: Evelake, Lady Thora, Prince Vselov, V, Jarl Bolthar, Lilith, and others.

Octopuses are the gods of this religion, but these eight-armed monstrosities pray to the Leviathan, who is their god.

Certain documents say that the Babylonian dragon Tiamat was either an agent of the Octopuses or an agent of Leviathan (or possibly an aspect of Leviathan) and the hero Marduk was able to defeat her with a magical spear. Certain other documents point to the first flood, recounted in sundry texts such as the Bible, the Koran, the Classic of Mountains and Seas, the Elder Edda, and many, many more, as being caused by the Octopuses and/or their agents.

The Tailed People are usually on the lookout for maidens with supernumerary digits, for the purpose of ritually sacrificing them to the Octopuses, one presumes.