Gates of the Moon

Gates of the Moon was a Celestial plane in the World Tree cosmology model and later an astral dominion in the World Axis cosmology model. Prior to its attachment to the World Tree, it was Selûne's realm in Gladsheim/Ysgard as described by the Great Wheel cosmology model.

Description
The Spellplague caused the Gates of the Moon to be cast adrift in the Astral Sea and Selûne shared this plane with some allied deities. One thing that all the cosmological models agreed upon was that the Infinite Staircase was based here.

Great Wheel Model
On the first layer of Ysgard, magic was altered such that most wizard spells required a school-specific rune to be inscribed on the material components, and if a spell had no material component, the rune had to be incorporated into the verbal component. A visiting wizard would have to learn the correct runes from the local inhabitants, or come by the knowledge some other way, in order to cast any spell other than strength or melee combat spells (i.e., enlarge, fly, haste, jump, polymorph, stoneskin, strength, etc. all worked as normal without engraved runes). Another quirk of this plane was that any summon monster spell only summoned einheriar. Divination, necromantic, and wild magic spells were enhanced in Selûne's realm, but weather-related spells that could cause the moon to be obscured failed utterly. Divine spellcasters did not have the runic restriction, but started to lose their higher level spells if their deity was far removed from Ysgard.

The ever-present moon waxed and waned, causing regular tides in the sea surrounding Selûne's palatial hall as well as shifts in the personality of the inhabitants. The brighter the moon, the more happy, friendly, and stable were the natives. As the moonlight faded, the locals became sullen, withdrawn, and even angry or hopeless.

World Tree Model
This plane was primarily a seemingly endless ocean of luminous liquid light under an equally boundless night sky full of stars and Toril's moon. The moon waxed and waned but was always present. In the middle of the ocean was a island of stone with Selûne's magnificent silver acropolis known as Argentil at its crown.

Travel to the Gates of the Moon could be accomplished in various ways:
 * The usual methods for mortals were by astral projection, plane shifting, or a gate spell.
 * For those with good connections, the celestial eladrins maintained a permanent portal to Arvandor to connect with the other large enclave of their race.
 * The upper branches of the World Tree reached this plane and could be used to travel to other Celestial planes if negotiations with the guardians of the Tree were successful.
 * The Infinite Staircase appeared every full moon inside Argentil and it was said to connect to every city in every plane and perhaps in every time. Traversing this mysterious conduit was known to be quite hazardous.

World Axis Model
In addition to the island that held Selûne's realm, there were other stony islands that floated in the shining sea and in the sky above as earthmotes. Each was naturally beautiful and contained dwellings from rustic cottages to graceful mansions. Sune's city of romance, Brightwater, was built around Argentil's island. The cycle of day and night was observed here, but the moon was usually visible regardless of its phase even during the daylight hours.

The Infinite Staircase survived the Spellplague and connected Argentil to cities across the multiverse, but reportedly most of these allowed travel in only one direction. Selûne was known to keep portals to the astral dominions of Arvandor, the Deep Wilds, Green Fields, and Toril's parallel world of the Feywild.

Inhabitants
Many celesital beings inhabited the Gates of the Moon. The angelic servants of Selûne were female planetars known as shards. The Lillendi, when not patrolling the Infinite Staircase, also made their home here. Many eladrin dwelled on this plane or the attached demiplane known as the Court of Stars. Most numerous were the petitioners of all the deities that had a realm here. These included the embodied spirits of sailors, female spellcasters, good-hearted lycanthropes, bards, explorers, wanderers, travelers, and mystics.

Many mortals also lived in or visited the Gates of the Moon. In the Year of Sundered Webs, –339 DR, at the moment of the Fall of Netheril, Selûne rescued the Netherese enclave of Selûnarra by transporting it to this plane, where it has resided ever since. Over the years, the Selûnarrans interbred with Selûne's servitors and their descendant aasimars inherited the floating city. Mortal lycanthropes occasionally made their way to this plane to be relieved of the burden of their condition. Here, by the grace of Selûne, werecreatures had complete control over their bodily changes and their mental state in any form. Chaos dragons were known to frequent this plane also.

Realms

 * Finder Wyvernspur, the Nameless Bard, moved his realm, Fermata, from Olympus/Arborea to the halls of Argentil. The sound of music filled his chambers and he decorated all of Selûne's walls with artwork made by his petitioners.
 * Gwynharwyf, the Whirling Fury, was known to wander about the Gates of the Moon and Arvandor when not leading barbarian hordes in battle. She did not have a specific realm to call home.
 * Lliira, Our Lady of Joy, became an exarch of Sune and moved here from the plane of Brightwater with her after the Spellplague.
 * Morwel, Queen of Stars, ruler of the eladrin, and her consort, Faerinaal, made their home in a timeless demiplane attached to the Gates of the Moon.
 * Selûne, Our Lady of Silver, moved her realm here from Ghadsheim/Ysgard   and named it Argentil.
 * Sharess, the Dancing Lady, became an exarch of Sune and moved here from Brightwater with her after the Spellplague.
 * Shaundakul, the Rider of the Winds, blew through all the planes, but called this plane home.
 * Sune Firehair created the romantic city of Brightwater here after the Spellplague. It was named after her previous home, the World Tree plane of Brightwater.
 * Tymora, Lady Luck, also moved here after the Spellplague, constructing seven earthmotes and connecting them by bridges. She named her realm the Great Wheel after her former gambling quarter on the plane of Brightwater.