Prior to the 4th Century, the Cult of Noctifer was primarily situated in Greater Ataea where the deity was heralded as the Embodiment of the Arctic Nights that fell there during the Winter, complementary to the deity Solaris who was seen as the Deity of the Rising and Falling Sun. This was until about circa 381 where the kingdom of Teram, the central hub, underwent Religious Reformation, converting to a semi-monotheistic pantheon where Solaris became Solaris-Soter (Solaris the Savior) and the Head of the Pantheon reconstructed.
This was largely due, some say, to a vision of the Reigning Monarch at the time. Whether or not this is actively true is up to debate, but it is well known that the Reformation of the Pantheon brought with it social changes. One of these was the Persecutions of the 4th Century under the Reigning Monarch. What was once conceived of as a Deity of the Evening became a Diabolical Antithesis to the Solarian Worldview and its members. This particular change is well recorded in the heresiologies of Temple Fathers est. 397-410. The first recorded use of Noctifer as a “Devouring Force” is found circa 398.
The Persecutions were dominantly in the Northern Regions where Noctifer had a role as the Arctic Deity, alongside his presumed ‘family’ of deities. It did, however, also move eastward towards the coast where Noctifer took on a role as a god related to storms, the depths of the sea and diseases of a cold or watery temperament (he is well-noted in some villages as a deity of herbalism, especially of poisons and the capacity of poisons to be used as medicines, not to be confused with the deity Zet, who was a local deity relating to the usage of ritual narcotics in the eastern region).
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We only have remnants of the texts of the Noctifer Cult, of which only a few are legible as of current. Most others are anecdotal, journal entries from those of the persecutions. Our only other sources are artifacts, items which either refer to or directly bear the name of Noctifer, but these are few and far between and less likely to be found anytime post the 4th Century, when such artifacts would’ve been discouraged and any prior ones fallen into disrepair or actively destroyed on the basis of ‘heresy’ and ‘sorcery’.
There is nothing linking Noctifer immediately to the underworld and even the Solarist Reformation doesn’t link him to the underworld or its facets, but it does give him a rather strange attribute as a psychopomp, though this is only regarded in the text of one Solarist Sect in Ataea-Minor.
“And Lo, the Shadowy Moon cometh over the abode of the Dying One, for whereas Fair Solaris guides the Soul upwards, does Dreadful Buthios guide the Shade downwards” -
(The Book of Seven Steps)
It is to be remembered that the above text is that akin to other funerary texts, meant to be as guide to the soul in its traversing between realms and so has rather mystic qualities.