Aerenel

Aerenel is an isolationist island nation to the south east of Khorvaire. The entire island is blanketed by a manifest zone to Irian, and this powers the Undying Court. Armed with this divine energy, the Undying Court has repelled even assaults from Argonessen. Outside of this manifest zone, however, the Undying Court loses much of it's power, and so the elves have chosen to remain in their island paradise, rather than expand.
It is from this vantage point that the elves watched the rise of the empire of Dhakaan, and likewise watched it's fall. They also watched the rise of Galifar, and observed it's collapse into chaos, and then separate states. The elves of Aerenel expect that they will still be watching when the Five Nations crumble.
Given the power of the Undying Court, it would be a reasonable assumption that Aerenel is ruled by the deathless. This, however, is not the case. The Sibling Kings rule Aerenel from the nation's capital of Shae Cairdal, and the high priests and lords are all mortal elves. Rather than rule over them as one might expect, the dead in Aerenel guide and protect the living.
Most of the deathless are tied to spirit idols, small objects that preserve the souls of the dead and keep them from passing on to Dolurrh. Others are raised as deathless, a form of positive energy powered undead. Over time, these deathless can even ascend to a higher form, becoming spiritual beings that have no need for a physical body. These spirits, which are known as Ascendant Councilors, are housed in Shae Mordai, the city of the dead, and when these spirits join together they form a mystical gestalt force that has repeatedly repelled assaults from all quarters, including mighty Argonessen.

Life & Society

Aerenel is a land steeped in magic of all kinds, and it plays a greater role in Aereni civilisation than it does in Khorvaire. Every citizen of Aerenel knows at least some magic, and they would likely consider Khorvaire's master magewrights to be apprentice-level trainees. Whereas Khorvaire is generally capped to 3rd level spells being widely available, in Aerenel, spells of up to 5th level are commonplace, and it wouldn't be unreasonable to find master wizards in Aerenel with access to 7th level spells. Their cities are connected by teleportation circles, and communication can occur instantly anywhere on the island via sending stations. This is apparent in every aspect of Aerenel, even their buildings are magically raised using spells like stone shape.
At a glance, it would very much seem that the people of Aerenel are just better at everything than people on Khorvaire, and for the most part, this is true. Aerenel is an island nation entirely populated by individuals who live for a very long time, they have multiple human lifetimes with which to practice their crafts. This is reinforced further by an intense work ethic, as only the most exceptional individuals can ascend to the Undying Court, and this is a key motivation in the life of every citizen.
Where Aerenel falls behind Khorvaire is in industry, and innovation. For the most part, the elves of Aerenel are stuck in their ways, they aren't open to experimentation and they aren't interested in learning new ways to do things. Instead, they are motivated by mastering the skills of past masters who they believe have already perfected their crafts. Unlike in Khorvaire, most of those old masters are still around, and still actively teaching their techniques! The artificers of Aerenel might be able to make things that are beyond House Cannith, but each and every one of them would be a handcrafted masterpiece that takes years to make, while the forges of House Cannith are churning out hundreds of items a day, and they are always improving. Khorvaire might be lagging behind Aerenel for now, but the last thousand years have seen Khorvaire develop significantly while Aerenel has remained relatively stagnant.

Government and Politics

Aerenel is effectively a collection of city states that are united in their devotion to the Sibling Kings and the Undying Court. Each region of the island (with the exception of the northern territory of the Tairnadal) is governed by a noble line. These noble lines reflect the original settlers of Aerenel, which were composed of a number of different cultures and environments all fleeing from Xen'drik and the apocalypse that the giants had brought about. Today, whilst each of these noble lines share common customs, they each have their own traditions on top of these.
A noble line is made up of several ancient families, and governed by a house that carries the name of the line itself. This means that, for example, the line of Mendyrian refers to multiple families, but someone with the surname Mendyrian is a member of the Mendyrian family specifically. Each noble line represents the best of their region, and despite what the name might imply, they are not hereditary. Instead, the noble lines of Aerenel are a meritocracy whose members are chosen from those living within a particular region. You are not born into nobility in Aerenel, you have to earn it.
Similarly, the rulers of Aerenel, the Sibling Kings, are members of the same noble line but aren't necessarily blood relatives. They are always brother and sister, but this bond can be symbolic rather than genetic.

The Tairnadal

Whilst the Aereni are the dominant culture on Aerenel, they aren't the only one. The Tairnadal also occupy the northern steppes of Aerenel, but while these elves share a common history with the Aereni and came to Aerenel with the other settlers from Xen'drik, they are a separate culture. The Aereni and the Tairnadal are allies, and when the island is threatened they band together to repel any threats, but they otherwise don't share customs.
Notably, whilst the Aereni seek to preserve their dead as deathless members of the Undying Court, the Tairnadal instead believe that living elves can serve as anchors to keep their ancestors from fading in Dolurrh, and they believe they can do this by emulating the deeds of their ancestors. It's possible for elves to migrate from one culture to another, and the noble line of Tolaern is well known for having close ties to the Tairnadal, but they are separate cultures. The Tairnadal don't worship the Undying Court, even if they undoubtedly benefit from it's presence on the island, and Tairnadal elves don't become deathless.

Type

Geopolitical, Country

Demonym

Aereni

Major Exports

Crafts, bronzewood, densewood, livewood

Population

2,650,000 (77% elves, 19% undying, 3% half-elves, 1% other)