Ruidus

Ruidus is the smaller and more distant of Exandria's two moons, considered its secondary moon. Its dull reddish color reflects less light than Exandria's other moon, Catha, and it appears tiny in size. This makes it difficult to see in the sky, even at night, during the half of the year it is considered prominent. Exandrian scholarship knows little about Ruidus and puts forward conflicting theories on its origins. Various folklore casts it as an ill omen.

Description
Ruidus is less than half the size of Catha and so is small in the sky. When the moon is full, it shines vermilion, but usually appears as a dull color, variously described as brown, reddish-brown, maroon, deep red, or purple. Its distance and color cause it to reflect less of the sun's light. As a result, even when Ruidus is considered prominent, it is often difficult to see at night and near impossible to see at day; it has greater visibility during certain portions of the night.

It takes either half a year (about 164 days) or "a near six month cycle" (which could be up to about 182 days) to orbit Exandria. In 836 PD, it was visible during the four months between 22 Misuthar and 26 Unndilar; in Unndilar, it was on the opposite side of the sky from Catha. Unlike Catha, it does not wax and wane, and it sometimes glows or flashes at seemingly random times. On some nights, Ruidus unexpectedly glows to a full moon or fails to appear in the sky.

Folklore
In comparison to Catha, Exandrian scholarship does not know much about Ruidus. The origin of Ruidus's existence is unclear; some believe that it is pre-Founding while others believe that the Betrayer Gods created it during the Age of Arcanum or the Calamity as part of a lengthy but unknown plot that was abruptly halted by the Divergence. The myth of Alyxian, the Apotheon, places the creation of Ruidus during the Founding. At that time, "an unidentifiable power" was "seeping through the fabric of reality." This leakage of power into the Material Plane presented a threat to the nascent life on Exandria. The myth holds that the "dark power...crystalized into Ruidus". As the gods remain unaware of power's origin, they "agreed to create a tale about Ruidus to conceal its alien origin from the mortals of the world, informing them that it was a moon of ill omen, and its magical influence was always to be avoided." The myth asserts that "the tale concocted by the gods was not a lie." Contrariwise, as of the 830s PD, scholars of the Netherdeep claim that the magic attributed to Ruidus derives from Alyxian.

Folklore across Exandria supplies the information regarding the symbolic association, religious significance, and phenomena connected to Ruidus. It is commonly believed across multiple cultures that Ruidus is an ill omen, and those born while it is bright are said bound for lives of ill fate. As of 836 PD, remote parts of Tal'Dorei secretly sacrifice children born under a full Ruidus, partly as an offering to the dark moon and partly due to their belief that such children will suffer under a lifelong curse. The practice is banned in Tal'Dorei's cities. There is a prominent superstition that meteor showers are parts of a decaying Ruidus that have broken off and fell toward Exandria.

Shortly after the discovery of a "strange mineral in the sunken ruins" of Cael Morrow beneath Ank'Harel by academics of the Allegiance of Allsight, members of the Consortium of the Vermilion Dream misattributed the mineral's origin to Ruidus and named it "ruidium" due to the resemblance of its red hue with the light of Ruidus. The color and properties of the mineral derive from magic which issues from the adjacent planar rift. Despite the misconception, the misnomer of "ruidium" remains the name for the mineral.

Magical phenomena
The light of Ruidus has no observed effects on entities with lycanthropy.

Extraplanar phenomena
Members of the Allegiance of Allsight below Ank'Harel in the 830s PD discovered "a gaping [planar] rift that pulsates with sickly red light" known as the Netherdeep. The myth of Alyxian connects the origin of the Netherdeep with his interposition between Gruumsh's spear and the city of Cael Morrow during the Calamity. An extant version of this myth holds that "the Apotheon's connection with Ruidus flared to life" at the moment of his interposition.

Some worry that rare double eclipses, where Catha and Ruidus simultaneously overlap the sun, cause the Material Plane and other planes to calamitously intersect.

Critical Role: Call of the Netherdeep
The introduction to Critical Role: Call of the Netherdeep explains that the story "intentionally avoids discussing [Ruidus's] actual magical power" and that "the true nature of Ruidus is a topic to be explored in other Critical Role stories." The adventure's narrative focuses on Alyxian, the mythological figure of the Calamity. The myth of Alyxian says that Alyxian was born under the full light of Ruidus. At the creation of the Netherdeep in Alyxian struggle with Gruumsh, the myth holds that "the Apotheon's connection with Ruidus flared to life" when he intercepts Gruumsh's spear. The myth of Alyxian and the veneration of Ruidus hold that "Ruidus's alien magic" is responsible for certain curses of ill-fate. Nevertheless, most acknowledge that the magical phenomena and the ruidium mineral connected to the Netherdeep derive their power from Alyxian. Consequentially, if the adventuring party successfully relieves Alyxian of his bondage in the Netherdeep, all the ruidium in Exandria ceases to exist.

Trivia

 * In the third chapter of Explorer's Guide to Wildemount, its name is misspelled "Ruidis".