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.

Though often depicted close to each other, Catha and Ruidus have different orbits and can appear in different points in the sky.

Appearance from Exandria
Ruidus is less than half the size of Catha and appears 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.

Geography and arcane features
Ruidus is known to have mountains, which are visible through a telescope from Exandria. The moon appears to have an atmosphere, as stormy weather patterns with crimson clouds and flashes of light are also visible through a telescope. There is also at least one city on its surface.

There is an apparent enchantment that prevents people on Exandria from looking too closely at Ruidus, allowing it to "evade" inspection. Viewing it from a telescope causes the looker to lose the ability to focus and ultimately induces a headache. It is also surrounded by a lattice-like arcane weave similar to the Divine Gate, visible through the Veilscatter Scope.

Orbit and flares
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. A flare occurred around 840 or 841 PD. It flared again in the middle of the night of Sydenstar 14th, 843 PD.

Folklore
In comparison to Catha, Exandrian scholarship does not know much about Ruidus; however, as of 843 PD, rising interest in Ruidus has led to the funding of new research. Exandria Unlimited: Calamity implies that Ruidus existed by the beginning of The Calamity.

The origin of Ruidus's existence is unclear; some believe that it is pre-Founding while others believe that the Betrayer Gods created it as part of a lengthy but unknown plot that was abruptly halted by the Divergence. Another myth asserts that an unknown power began to bleed through reality during the Founding, and that the gods recognized that it would threaten all life. They worked together to banish it, but evidence of the incursion remained on the Material Plane in a crystallized, reddish form. Per Call of the Netherdeep, the gods seeded the folklore associating Ruidus with bad luck in order to hide its origins and to encourage mortals to avoid it. However, Ruidus truly does influence the fates of people who are born or go adventuring during a flare.

It is commonly believed in folklore across Exandrian 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 red mineral in the 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". The color and properties of the mineral actually derive from magic which issues from the adjacent planar rift. Despite the misconception, the misnomer of "ruidium" remains the name for the mineral.

Ruidusborn
Individuals born while Ruidus is glowing or flaring are said to be Ruidusborn. Some are stigmatized for being Rudiusborn while others believe that it simply means the individual is somehow fated, for better or worse. The following are individuals known or thought to be Ruidusborn, listed chronologically:
 * The Raven Queen
 * Alyxian
 * Warren Drassig
 * Liliana Temult
 * Imogen Temult
 * Fearne Calloway

Scholarship
Scholars whose focus pertains to Ruidus are few, and study of Ruidus has been difficult because something either about the moon itself or in the sky above Exandria prevents the ability to deeply investigate it or ascertain its nature using magic. Looking at the moon too closely through telescopes causes the looker to lose the ability to focus and ultimately induces a headache. This tendency to "evade" understanding and study has been noted in research for years, but researchers do not openly discuss this quality to avoid stirring anxieties related to popular superstition about Ruidus.

In 836 PD, Demid Sunlash explained that his fascination with Ruidus is due in part to how little scholarship exists. In the summer of 843 PD, Roe Estani mentioned that a growing recurrence of Ruidus flares and curiosity regarding the "abnormal record" of Ruidusborn had led to an increase in funding. The Aydinlan Seminary in Yios, Marquet is associated with Ruidus studies, given its apparent resources on Ruidus in the Omen Archive and the faculty research activity of Kadija Sumal and Estani, though Estani did not publish his later findings on his theories regarding Ruidus. Balan and Zeyar Lumas also took special interest in the sporadic flares of Ruidus, and they were traveling to the Omen Archive at the Aydinlan Seminary in Yios, Marquet to pursue their research but were murdered en route. Members of the Grim Verity, including Estani, Balan and Zeyer, and Hondir, have researched Ruidus, and many of them were murdered for their interest in it.

Magical phenomena
On Twitter, Matthew Mercer stated that light of Ruidus has no effects on those with lycanthropy in comparison to Catha. In-game, werewolf Gurge Kisgregg remarked that "Catha drives it mostly" but darkly added that "everyone's kind of at a disadvantage" when Ruidus is at its peak, apparently in reference to Ruidus' as a sign of ill omen. However, Chetney Pock O'Pea's memories suggest a connection between his lycanthropy and Ruidus, though Matt subsequently confirmed to the table that Chetney's lycanthropy is driven by Catha's full moons rather than Ruidus.

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, and attributes its creation in part to Alyxian's connection with Ruidus.

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

Call of the Netherdeep
Ruidus's role in the story of Call of the Netherdeep and its true nature are kept deliberately vague, but it is stated to be tied to the fate of those born during its flares, such as Alyxian. A Ruidus flare is among the possible encounters the adventuring party may experience as they travel to Bazzoxan.

The description of the flare, in contradiction with other statements, implies that Ruidus does wax and wane, as it is described as appearing as a "slim crescent", suddenly becoming full during the flare.

Campaign 3
Imogen Temult, in researching strange recurring dreams she had been having in which she was attempting to escape a red storm, found a paper that indicated such dreams may be linked, among other things, the cycle of Ruidus. She later, with Orym's help, looked at Ruidus through Estani's telescope, and saw that a storm similar to the one in her dreams raged on the surface of the moon.

While in Bassuras, Imogen's dream changed; rather than trying out outrun the storm, she found herself within it and that there were armored figures standing before her. When she woke up, she was drawn towards the window, and upon looking up at the sky saw that Ruidus was flaring.

Birdie Calloway revealed to Bells Hells that Fearne was born in Exandria during a Ruidus flare. She and Fearne's father, Oleander Calloway, were unaware of the significance, as Ruidus did not appear in the Feywild. Later, while under the influence of herbs, Ollie had a vision of Ruidus appearing in the Feywild and destroying it utterly. A year after that, they saw Ruidus in the sky of the Feywild. They began to investigate, and found that the Unseelie Court was concealing Ruidus's influence, and was working with people from the Material Plane, including Otohan Thull, on a machine related to Ruidus. She and Ollie stole the Moontide Crown from this device to create their own machine in Exandria to study Ruidus with the help of Ira Wendagoth.

Upon the completion of this device, known as the Veilscatter Scope, Imogen looked at Ruidus again. This time, she was able to see a lattice-like weave encasing the moon, which she told Ira was a prison for a god. After Orym and Ira helped focus the scope further, she was able to see a city on the moon, visible through the storm clouds. Afterward, Hondir identified the lattice as similar to that of the Divine Gate.

Trivia

 * In the third chapter of Explorer's Guide to Wildemount, its name is misspelled "Ruidis".
 * References to Ruidus being made of cheese:
 * As part of a ruse, Caleb Widogast tells Demid Sunlash that Whitestone myth and legend understands Ruidus to be made of cheese. Sunlash concludes that this is "strange" and remarks that "Tal'Dorei is such an uncultured land".
 * Ashton Greymoore states that they "grew up thinking the red moon was made of cheese."