Excerpted from The World of the Wilderlands of High Adventure:

Ioun has an eccentric orbit that passes from within the orbits of Gorm and Luna out beyond those circuits to the nearest reaches of Heaven’s Bridge, completing one orbit every 90 days. At its furthest it appears to be no larger than any of the countless stars in the sky; at its closest, it appears five times larger than the nearest planet, about the size of a thumbnail at arm’s length. Its coloration is seemingly random, and dances through the full spectrum by the hour or the day, sometimes into wavelengths not visible to mortal eyes. When Ioun is closest to Gaia, stars fall in profusion from the sky, and often these meteorites have Ioun stones at their core or spawn monsters through their proximity. The most powerful, unusual, and dangerous stars fall when Gorm and Luna are full or new in unison while Ioun is at perigee, a very rare occurrence. Sages believe that Ioun touches Heaven’s Bridge when it is furthest from Gaia, and the stars that fall are pieces of the bridge that broke off during the collision. As Ioun is often quite important in major arcane rituals, it has gained the epithet “Wizard’s Moon,” and thus is associated at times with gods of arcane wisdom and power such as Bachontoi, Dyrantil, Hercon, Myrddyn, Tar-Ark, and Vicon.

This moon is also of especial interest to the elves and followers of Cilborith, for they measure their calendar by its revolutions.