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.

Study and mythology
In comparison to Catha, Exandrian scholarship does not know much about Ruidus and much that is known or believed comes from folklore. 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. 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. Additionally, there is a prominent superstition that meteor showers are parts of a decaying Ruidus that have broken off and fell toward Exandria.

It has no effect on lycanthropy. Some worry that rare double eclipses, where Catha and Ruidus simultaneously overlap the sun, cause the Material Plane and other planes to calamitously intersect.

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.

Behind the scenes
The influence of Ruidus plays a role in Critical Role: Call of the Netherdeep: "the red moon of Ruidus watches, twisting the fates of those who have the power to shape the course of history."

Trivia

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