The Heroic Domains of Ysgard is a Chaotic Neutral (Good) plane in the D&D Great Wheel Cosmology.
Diametrically opposite of Acheron; the two planes have many things in common: they are both planes of conflict and strife, the landscape is in constant motion and turmoil, and rather than playing home to one specific archetypical inhabitant, the planes are filled with a variety of races and belief systems who have cause to come into conflict with each other.
However, in Ysgard life is better because there are rich rewards and satisfaction for what an individual can accomplish, unlike Acheron where one successful conflict just leads into another one. The enemies in Ysgard often have legitimate grounds for fighting one another, rather than contrived or forgotten reasons. Also in Ysgard, nothing is permanent, not even death. So while conflict and violence are commonplace, nothing is outright malicious. However, being dead before you get here is no excuse; the magic won't resurrect those who haven't been killed on the plane.
Just remember that despite being on the upper side of the wheel, it is not a plane of good. The plane does not exist for those who desire to enhance their personal growth or to contribute anything to those around them; those souls would be more likely to go to the neighbouring plane of Arborea. Ysgard is for those who can never be content with their lot and just want to pit themselves against something new and challenging, and thankfully the plane can provide that.
The plane is where dead heroes go, it is essentially Valhalla, the Viking plane of the afterlife. Many who come here get to drink all night and fight all day. At the end of the day, the plane's magic resurrects those slain in the day's fighting, and they all go in to get drunk and tell stories.
Again, much like it's opposite of Acheron, the landscape is an infinite void filled with floating matter that constantly moves around. The main difference is that land mass in Ysgard has no definable shape or permanency. The earth flows like a river across the void, constantly changing direction and topography. The only permanent features tend to be "inland" where the earth is less likely to take a radical change of direction and destroy what has been built.
Some land masses float entirely separated from the others, forming "earthbergs" that look like floating islands. These islands or even the edges of continents can flow into each other, causing new mountain ranges to form. Or they pull apart, leaving empty space in the ground which can lead to the layer underneath.
The first layer is actually called "Ysgard", and it is the most populous of the three layers.
Ysgard itself has an appearance easily identifiable with the material plane, though with a lot more vibrancy and energy. The weather tends to be a lot more spectacular and the landscape is far more rugged and inhospitable. With the constant motion of the ground there are few permanent settlements, with the inhabitants of the land living in mobile camps or frontier towns with the occasional grand hall built at places of interest. Only the deities really have the power to fix the terrain into anything truly permanent.
Though with that being said, there is the great city of Himinborg, which houses the largest population in the layer and likely the plane, which sits in the Plains of Ida, a large open expanse which holds daily festivals and markets. Though most later editions ignore the connection with the Norse Pantheon, Himinborg is actually the divine realm of Heimdall, and the city sits at the gates of the Bifrost, which can be directed to touch any world of the Prime Material Plane.
The layer is also littered with battlefields, where armies have come together to fight one another for whatever reason.
A large region of the layer is called Aflheim (Elf-Home) where funnily enough, there lives a significant presence of elven petitioners and mortals. Aflheim has its own weather system and has a recognisable pattern of seasons, where in winter it gets so cold and bitter that the Elves have to retreat into caves to keep themselves warm.
The second layer, literally underneath the first layer, is a realm of fire and hostility. The land has the fire-dominant trait and is replete with volcanoes and lava streams, and like the first layer the ground is in constant motion. If you're not fire-retardant you're going to be in trouble.
Fire Giants and their deities make their homes here. Sometimes, an earthberg flips over by accident and floats around the other side flaming-end-up.
The bottom layer, unlike the other two is an infinite realm of earth, rather than floating freely in the void. That doesn't make it any more stable and it still flows and clashes like everything else in the plane. Therefore travellers should be wary about trusting any maps as caves and tunnels may have caved in or changed location over time.
Nidavellir is home to several dwarven and gnomish realms, who constantly compete for the precious resources that get discovered as the layer flows over itself, so they often end up in conflict with one another. Though they also find common cause fighting the Dark Elves who make their homes here in the realm of Svartalfheim (Black Elf home), although ironically, the Drow who live here tend to be of chaotic neutral or chaotic good alignment, since that is the alignment of the plane and generally of its inhabitants. Evil Drow tend to end up somewhere else (like hanging with Lolth), so the Drow living here would rather be left alone rather than capture and enslave you, but they fight when they have to. So you end up in sort of a three way arms race, where the weapons and spells get ever more spectacular as time goes on.
The Petitioners can be of any race or culture, as long as they either fit the alignment or worshipped a deity that holds a realm here, that's part of why there is some much conflict on the plane (even though that's usually true of all afterlife planes). They appear as they did in life, there is no inherent requirement for them to change into animals or celestials, and that's pretty much the point. They come to this plane as individuals and they intent to make their mark. Though that said, they aren't given any particular special abilities either, and since they lose all of their memories at the time of death, they may not necessarily be extremely powerful or dangerous... But they love it here and that's what counts. Plus there are lots of them, and you can't kill them permanently, so there's always that.
Eladrin also live here in large numbers, since this is a chaotic plane that borders Arborea that should be no great surprise, though you should expect them to be a bit more rough around the edges here in Ysgard.