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 vermillion, 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. Folklore across Exandria supplies the information regarding the symbolic association, religious significance, and phenomena connected to Ruidus. It is unclear when Ruidus came into existence; some believe that it is pre-Founding while others believe that it was created by the Betrayer Gods during the Age of Arcanum or the Calamity as part of a lengthy but unknown plot that was abruptly halted by the Divergence. 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 early 830s PD discovered "a gaping [planar] rift that pulsates with sickly red light" known as the "Netherdeep." The myth of the Apotheon connects the origin of the Netherdeep with the Apotheon's 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
Critical Role: Call of the Netherdeep focuses on Alyxian, known as the Apotheon, the mythological figure of the Calamity. While the association of Ruidus with turns of ill-fate for plans made or those born under its full light is significant, the magical phenomena and mystical lore that groups such as The Consortium of the Vermillion Dream claim "to receive from the vermillion moon are related to Ruidus only so far as people believe them to be." 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. While the magic of Ruidus in Critical Role: Call of the Netherdeep may be the fashioning of folklore, the symbolism of Ruidus is a theme of the narrative. Adventurers who answer the call from the Netherdeep enrich their understanding of the role of Ruidus in Exandria's cultures across history.

The myth of the Apotheon says that Alyxian was born under the full light of Ruidus. At the creation of the Netherdeep in the Apotheon's struggle with Gruumsh, the myth holds that "the Apotheon's connection with Ruidus flared to life" when Apotheon intercepts Gruumsh's spear. The myth of the Apotheon 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 so-called "ruidium" mineral connected to the Netherdeep derive their power from the Apotheon. Consequentally, if the adventuring party successfully relieves the Apotheon 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".