The Verdant Fields of Arvandor was a notable layer of the Chaotic Good plane of Arborea in the Great Wheel cosmology of Dungeons & Dragons, as well as being a stand alone plane in the World Tree cosmology of the Forgotten Realms campaign setting of 3e. It was later a Dominion within the Astral Sea of the World Axis cosmology of 4e. In all incarnations it is the home of Corellon and Sehanine, as well as their servitors: the Seldarine. In the World Axis, it is also a common resting place for Avandra and Melora, who have a particular compatibility with Arvandor and its owners.
Great Wheel / World Tree Version
Arvandor is practically the same in both The Great Wheel and World Tree cosmologies; mechanically both acted alongside one another in 3e, depending on your campaign setting. Though even Forgotten Realms has switched from the Great Wheel to the World Tree and back again across editions; the World Tree version existing as the divine realm of the Seldarine, while the Great Wheel version is the discrete realm of the Seldarine located within the larger structure of the plane. But for all intents and purposes they are the same thing.
The plane/layer has two names: Arvandor proper, which is the domain of the Elvish pantheon, and Arvanaith which is the realm of the elven dead, though they are often held to be the same place.
Arvandor is an infinite expanse of deep woods and unearthly beauty. It is so caught up with the Chaotic Goods passion of Arborea that the layer practically exemplifies the entire plane unto itself. If one forgets that the Olympian pantheon also shares proximity to them in Arborea, the plane might be considered the sole property of the Elves and Eladrin that make their homes here.
One thing is not well known however; Arvandor was not always the home of the Seldarine. It is believed that the Elven gods won the realm from the Giant Pantheon at some point in unremembered history, Giant ruins still litter the plane, forever acting as a reminder of the former inhabitants.
Despite its beauty and good natured intent; Arvandor can be a perilous place to live. Dangerous beasts inhabit the forests and plains, requiring the planar powers that be (whether divine or mundane) to send out knights and hunters to regularly cleanse their nearby regions and make them safe again. Additionally, the threat is not limited to unthinking beasts, as the Unseelie Court also make inroads to the plane and wicked Fey creatures can snatch individuals who drop their guard for too long. This makes the realm somewhat hazardous for an upper plane, though the Elven petitioners and their Eladrin allies maintain constant vigilance ensuring that Arvandor remains secure.
Arvandor has many links to other places, thanks to the nature of its pantheon. The divine powers that be also shape the region to their whims and create a varied landscape when not allowing wild nature to run rampant. Corellon and Sehanine live together in a magnificent marble tower called Gwyllachaightaeryll which means "the Many-Splendored"; also called "Overlook". The tower rises and twists through an area of trees known as the "Crescent Grove". There are many rooms in his tower and it reshapes itself according to his whim so that no room appears the same way twice, with the exception of his throne room.
The region of Elavandor is the ocean domain of Deep Sashelas, which is either a part of the layer of Ossa or merely a divine realm nestled within Arvandor's Sparkling Sea. This even changes within editions and campaign settings, because Sashelas constantly reshapes his environment according to his whims, raising new islands and coral reefs as he is never satisfied with his accomplishments and continually seeks to improve on his works. His palace usually sits at the bottom of a deep chasm, although the water is so pure that light from the surface still manages to filter its way to the bottom, in fact the only time the realm darkens is when Sashelas leaves. Here, any good-aligned elf can breath water as if it were air, allowing any good-natured Elven visitor to approach him if they feel the need too.
Evergold is the Fountain of Youth, which exists on multiple planes at once and is simultaneously located in the realms of Hanali Celanil, Sune, Freya and Aphrodite. On the Elven side of the nexus of planes is a crystal palace made of one-way mirrors, where anyone inside can look out but cannot be viewed.
Speaking of overlapping regions, Aerdrie Faenya's floating tower exists so close to the philosophical borders of chaos that the realm freely hops back and forth between Alfheim in Ysgard. The Elven goddess is so chaotic that her realm is a region of storms and howling winds, though this usually gets toned down on the Arvandor side of the divide, but in either case the tower is practically impossible to reach except for the most determined of flying individuals, who then have to get past the Avariel guards who aren't keen on visitors; this is all despite Aerdrie being a "good" goddess, it is simply that she has very little interest in the affairs of others and has few relations with other divinities to allow her doors to remain open to visitors.
Other lesser or intermediate deities have their realms, but are typically transitory, rarely staying in one place for too long as befits deities of chaos.
World Axis Version
Arvandor takes the form of six large, steep islands covered in glorious fey forests, with its border islands forming an archipelago of dozens of smaller, but no less beautiful and verdant, isles. Many poets like to wax rhapsodic about its eternal beauty and the life of ease and luxury that comes to the ruesti - the Exalted chosen to dwell here. Such poets are typically humans or halflings who don't actually worship Corellon, or else they'd know they're talking out of their asses.
See, Arvandor is a beautiful place. But, much like the Feywild - and there are many portals and rifts connecting the two - it is a rugged, dangerous beauty. Under the enchanting glamors lies a world of constant danger, wild battles, and an eternal call for battle, as neither of its creator-masters believes a life without loss is worth living. An endless succession of peaceful, perfectly happy days sounds more like purgatory than paradise to those truly called for Arvandor.
Furthering this, Arvandor's creators objected to the Carceri Compact, arguing that no prison could ever be made truly escape-proof, even one forged through the powers of all gods, and that if they were to try, the presence of Carceri in the Astral Sea would poison and corrupt the Plane Above. Finally, as a compromise, they agreed to make Arvandor into a "vent" for Carceri; one-way portals from the Prison Plane allow the abominations to escape into Arvandor, which means that it is the duty of the natives to hunt them down and slay them.
The Glorious Hunt, as it's called, is fraught with danger. Simply being an Exalted does't mean you have great fighting prowess, which is why the native Exalted train ceaselessly to hone their war-skills. Furthermore, as part of its connection to Corellon and Sehahine's beliefs, deaths on the Glorious Hunt are more "serious" than normal for an Exalted. The ruesti can only perish in such a battle three to a dozen times before their existence comes to an end, passing on to whatever fate awaits a truly-dead Exalted. To say nothing of how some of the more terrible abominations have the power to flat-out destroy souls.
Ironically, despite the lengths of their lives, for many an elf or eladrin, their afterlife is incredibly short. As far as they're concerned, though, it's well-worth it; beauty and splendor balanced by the need to treat every day as if it may be their last certainly beats an eternity of tedium.
The Glorious Hunt is so well-known that not only do Exalted of Melora and Avandra often take part, but so do Exalted or epic-tier worshippers of other gods as well. Kord and Bane worshippers love to test their mettle against the monsters of Arvandor, whilst the faithful of Pelor enjoy battling the children of darkness. Even faithful of Zehir have been known to take part in the Glorious Hunt, relishing the chance to pit their poisons and murderous arts against some of the most lethal creatures in the multiverse.
Arvandor is roughly divided into two regions; the Hunting Lands, the untamed, monster-infested wilderness that makes up the majority of the Dominion's territory and where the abominations escape from Carceri, and points of civilisation, where the more conventionally pleasant aspects of life in Arvandor can happen. The former includes sites like the Dead City of Carantharas (a fallen ruesti city left to the wilderness rather than reclaimed) and the Reef Catacombs (a flooded forest of enormous petrified trees, lost since before the Dawn War), whilst the latter includes sites like the Bark Fleet (a trinity of city-sized ships woven from living trees), the Court of the Seldarine and the Empire of Estaira (a massive city of mortals who have chosen to settle in Arvandor and prove themselves worthy of life here).
The border islands of Arvandor are known as the Green Isles. Although not bedeviled by monsters to the same extent as Arvandor, life here isn't all idyllic peace. Pirates and lesser monsters abound, as do agents of dark powers. The Twisted Temples of Lyef Thierre are particularly infamous for the mad, blood-thirsty cultists who have reigned over the island for countless generations.