"But those who do what is true come to the light, so that it may clearly be seen that their deeds have been done in God."
- – John 3:21, NRSV
It is THE place of ultimate goodness; while the peak at the seventh layer of Celestia can lay claim to being the most perfectly good and harmonious state in existence, Elysium on the whole is given to universal goodness without regard for law or chaos. The Best Place.
The place is a paradise, not given over to glory and triumph like the lawful heavens are, or given over to personal achievement and growth like chaotic good planes are. Here nobody needs to work unless they want to, nothing is difficult and you're always going to find a sympathetic ear and a helpful hand.
Elysium is not dangerous unless you come to the plane with ill intentions, in which case the petitioners and good outsiders that live here will fuck your day up. But it might not even come to that.
You see, Elysium makes it difficult to actually perform wicked actions; it is strongly good aligned for a start, so anyone travelling here who is not good aligned themselves will suffer mental penalties (yes, even neutral people). Not only that but the plane itself influences the senses, making sights and colours more wonderful, sounds more melodious and smells more pleasant. This influence is so strong that it even makes interactions between individuals feel more empathetic and understanding. The longer you spend in Elysium the less inclined you are to want to leave, eventually becoming trapped by the inherent goodness of the plane. From there you get to live out the rest of your life in peace until you die of old age and then continue living as a petitioner.
This effect does not change your alignment, but it does slowly drain you of your memories as you slowly become overwhelmed by feelings of tranquility. So you could possibly still be an evil bastard at heart, but you'd have forgotten all of your just as planned machinations and any of the wicked schemes you've prepared.
The final defense of the plane, is that it has a sort of "life" all to itself. Travelers of a good nature find the paths through the landscape make the journey more pleasant for them, whilst those evildoers who manage to get past all of the celestials and still have willpower enough to resist the tranquil aura that the plane emanates will find that the landscape frustrates their efforts, by leading them round in circles until they become lost and either are forced to leave the plane or succumb to its good nature.
One last thing to remember is that the plane has a minor positive trait, meaning that even if you managed to cause injury to somebody, their wounds would heal within minutes. While that doesn't allow for resurrection like on some other planes, it's just one more obstacle to overcome for those who come here determined to cause harm... and that's to say nothing about the realms of the goodly deities that live here and the modifications they can make to the traits of the plane.
The top layer would be deceptively similar to the material plane, with the landscape taking a recognizable topography. There are regular seasons and weather patterns, but nothing uncomfortable. That being said, the celestials don't like strangers dicking with the weather via magic, you don't need to mess with how the plane works.
The layer is absolutely typical of the "living" nature of Elysium, despite being a place of perfect goodness, misfortunes do still happen. The plane will regularly throw up events like accidental fires or children trapped down wells, but never maliciously and only very rarely do people get injured (they heal quickly afterwards anyway due to the positive trait). These occur in order to continually foster the community spirit, particularly for strangers to the plane where it is presented as an opportunity to test their better natures and get them involved, eventually leading to their inevitable entrapment on the plane.
A handful of divine realms are located here, these being: Release-From-Care, realm of Lydia that also serves as an important trading town, Principality, realm of Berei, Light's Blessing, the original realm of Pelor, Contemplation, realm of Majere, Healer's Home, realm of Mishakal, Quietude, realm of Isis, Valorhome, realm of Kuan-Ti, Domain of the Day-Long Sun, realm of Savitri, and City of the Star, realm of Ishtar.
Eronia is a mountain range of rough terrain and occasionally harsh weather. Rockfalls, lightning storms, uncomfortable summers and bitter winters; life does not appear easy for the petitioners, but it's probably only because they want it that way. Eronia exists for souls who still want a challenge in the afterlife, but the layer remains part of Elysium, so no-one is ever in any real danger and time spent here is never tragic or depressing.
The divine realms here are Dweomerheart, realm of Mystra, Morninglory, realm of Lathander, Ushas, Zodal, and Atroa, The Great Mother's Garden, realm of Chauntea, Liu, and Djaea, The True Grove, realm of Eldath, Soulearth, realm of Urogalan, Woodhaven, realm of Hiatea, Great Mountain of the East, realm of Enlil, Harphearth, realm of Bragi, and Mirror of the Moon, realm of Tsukuyomi. The sky god Velnius also wanders this layer, keeping no stable realm.
The least populated of Elysium's layers, and possibly the most dangerous. Belierin is a fog-filled swamp that appears at odds with the nature of the plane. There isn't a lot of travel on the layer and the Guardinals are especially suspicious of strangers moving around freely here. The reason for this is that it is believed that Belierin is used as a prison for powerful evil creatures, which are brought here to keep them contained by the confusingly good aura of the plane.
The celestials do regularly engage monsters that stumble their way out of the fog, therefore it is probably the only place in Elysium where the inhabitants might be in any real danger. It is still a place of ultimate goodness and there are towns built around gigantic lighthouses.
Only one divine realm is located here: Refuge of Night, realm of the goddess Nut.
One interesting difference about Thalasia compared to the rest of the plane is that those mortals who succumb to the inherent good aura of the plane and become trapped are permitted to retain their memories and their powers, they simply have no more desire to leave and are at peace with themselves. This means that Thalasia is an attractive place to journey for retirement where research can continue in peace or unfinished works be given more time to complete. It also makes the layer a haven for very powerful beings and a good recruitment grounds for souls that can be promoted into more powerful celestial creatures.
This layer is an endless ocean stretching off into infinity, but unlike Aquallor, the landscape is dotted with island realms of various gods or noble heroes. Even petitioners who want to spend some time alone can make themselves an island home here. Realms located on these islands include: River Amiel, realm of Myhriss, Fortress of the Sun, the realm Pelor established during the Greyhawk Wars, Selkie's Grotto, realm of Surminare, The Waves of Grief, realm of Nesirie, Isles of the Blessed, realm of Belenus and Brigantia, and Citadel of the Sea, realm of O-Wata-Tsu-Mi. Trishina can be found here when not with Deep Sashelas.
Thalasia is also the source of the River Oceanus which is like the good version of the River Styx, the water that flows from here will cross it's way back through the layers of Elysium and down through the Beastlands and Arborea where it will eventually be deposited in the endless ocean of Aquallor.
Elysium might as well be Narnia in that it is populated by anthropomorphic animal-celestials called Guardinals whose leader is a celestial lion called
Aslan Talsid. The celestial lion and his companions don't have a court or kingdom like the Archons or Eladrin do, and instead they just wander the plane doing good deeds.
Petitioners that arrive in Elysium don't take on any particular forms or shapes, they appear almost exactly as they did in life albeit considerably more chilled out. Interestingly, Elysian petitioners retain memories of their past lives and can keep up to four character levels from whichever careers they had before they passed away. This is in contrast to those mortals who get trapped on the plane and lose their memories instead.
Beyond the outsiders that live here, you can find all sorts of good races depending on which deities also make their homes here.