Arborea

Olympus, later known better by its elvish name Arvandor, was the Outer Plane in the Great Wheel cosmology model embodying the disorganized but well-intentioned motives of the chaotic good alignment. Although the gods of Olympus and Toril occasionally interacted, the people of Toril rarely concerned themselves with the affairs of the Olympians and vice versa. Memories of Olympus eventually faded and by the time the World Tree cosmology was introduced Arvandor was the accepted name for this plane. Some characteristics and deities originally belonging to Olympus/Arvandor were ascribed to the World Tree plane of Brightwater. Arvandor survived the Spellplague and became part of the World Axis cosmology.

Description
This plane had three layers with unique landscapes:

Olympus/Arvandor
Two realms, so vast and influential that their names became synonymous with the plane and the layer they occupied, coexisted with little conflict: Olympus, home of the Greek pantheon, and Arvandor, domain of the Seldarine. Even before the World Tree cosmology became popular, the influence of the Greek pantheon was primarily focused on other Prime Material Planes and had minor contact with the Forgotten Realms. Indeed, the two domains were separated by thousands of miles/kilometers of unclaimed wilderness and, due to the slight curvature of the landscape, were not visible to each other despite being located at the highest peaks of their respective lands.

Dominating the Greek-controlled realms was Mount Olympus, a towering edifice that served as home to many of the Greek gods as well as a multiplanar conduit connecting Olympus directly to Hades, Gehenna, Tarterus, and Alternate Prime Material Planes without passing through the Astral Plane. The usual color pools existed for Astral travelers and the permanent portals to neighboring Outer planes Happy Hunting Grounds/Beastlands, Gladsheim, and Concordant Opposition took the form of large, spinning crimson disks. Divination was required to determine the destination of each portal and all pools and portals within thousands of miles/kilometers of the Greek or elvish demesnes were either guarded, surrounded by stone walls and iron gates, or both.

This layer was starkly mountainous with deep chasms, huge passes, and foothills the size of Prime plane mountains. Giant trees adorned the rugged terrain while the flatter parts were covered with untended vineyards, orchards and fields of wild wheat, eventually giving way to wilderness. Monsters and evil beasts took advantage of the lack of vigilance and roamed the wild places, providing a challenge to any who would go exploring.

Ossa/Aquallor
Ossa to the Greeks and Aquallor to the elves, this layer was mostly filled with shallow seas no more than three feet (one meter) deep. The river Oceanus had its outlet here, delivering travelers from Elysium or the Happy Hunting Grounds/Beastlands safely in the middle of nowhere. The shallow seabed was occasionally broken by cavernous trenches leading to undersea domains of various sea gods or huge maelstroms that funneled water back to Thalasia in Elysium, thus completing the cycle. The barriers between layers of this plane were somewhat rare compared to other Outer planes. Being the middle layer, Ossa/Aquallor had barriers that led to the watery realms of Poseidon and Deep Sashelas in Olympus/Arvandor, as well as a few that lead to the third layer, Pelion/Mithardir.

Pelion/Mithardir
Mithardir means white dust in the elven tongue, and the mysterious third layer was a land of blowing white sand, with snow in the colder regions. Both the Greeks and the elves had stories and legends about powerful beings who once lived here, creating artifacts and architecture that became all but buried in the dust after they presumably died or departed. Their fate, and that of any creatures left behind, remains unknown.

Inhabitants
Devas,, hollyphants, planetars, solars, foo creatures, and agathia formed the usual cohort of creatures ubiquitous to the Upper planes. But Olympus/Arvandor was also the home of giant beasts and legendary monsters, such as giant bears, eagles, wolves, serpents, boars, lions, and wild cattle. Gorgons, cyclopes, giants, medusae, chimerae, harpies, sphinxes, and pegasi all made nests or lairs in the mountainous terrain or lived and lurked in the wilderness of the first layer.

Realms
Nearly all the known realms of this plane existed on the first layer.
 * Aerdrie Faenya, Seldarine Goddess of Air and Weather, had no permanent realm here but wandered among the layers of Arvandor and Gladsheim accompanied by dozens of huge birds.
 * Corellon Larethian, chief deity of the Seldarine, once had a realm here.
 * Deep Sashelas, sea god of the Seldarine, inhabited the waters that bordered the other great elvish realms. Some sages put his realm in Aquallor.
 * Erevan Ilesere, Seldarine God of Mischief and Change, once made his home on this plane.
 * Hanali Celanil, Seldarine Goddess of Romantic Love and Beauty, had a crystal palace here that surrounded an enchanted fountain which fed the magic pool Evergold (called Canathas by the Greeks).
 * Labelas Enoreth, elvish God of Longevity and rival to Erevan, once had a realm here.
 * Lliira, Goddess of Joy, Carefree Feeling, Contentment, Release, Hospitality, Happiness, Dance, and Patron of Festivals, once had a welcoming realm in this plane.
 * Nephthys of the Mulhorandi pantheon had a large palace carved from rock which blended in with the desert far away from the demesnes of the Greek and the elves. Some sages put her realm called Amun-thys in Pelion/Mithardir.
 * Rillifane Rallathil the Leaflord, whose realm was also his body, once put down his roots on this plane.
 * Solonor Thelandira, God of Archery and Hunting, had a realm of thick forests and rapidly flowing streams here.
 * Sune, human Goddess of Love, Beauty, Passion, and Charisma, once made her home on this plane. She shares the waters of Evergold with Hanali Celanil.
 * Tymora, human Goddess of Good Fortune, Luck, Victory, Skill, and Patron of Adventurers and Warriors, once had a realm in Arvandor.