There are only a few known methods to enter the Spirit Realm, with one of them being the use of Rift Magic, a ritual that allows those involved to astral prospect as spirits, entering the lifestream and passing into the Spirit World. However, leaving the Spirit Realm is considerably easier - it is commonplace for wandering spirits to enter the physical world.
The Spirit Realm operates on a different set of rules than the physical world. The world is seemingly infinite in every direction, geographies vary radically and often morph on whim, and gravity is not always constant. A common sight in the Spirit Realm is the physical manifest of the lifestream in the form of tangible, spiraling streams of energy gliding around landscapes and horizons. Stars may be identified by locating points of great spiritual energy, often where points in the lifestream intersect.
The realm is populated by both humanoid and non-humanoid spirits. Human spirits are usually spectral images of bygone souls, bound to the physical world by unresolved strife or internal anguish.
While seemingly infinite in scope and with a maze-like geography that's constantly changing, the Spirit World is defined by eight realms. These are:
Yggdrasil, Tree of Convergence
The centre of the Spirit World where the flow of life-stream ultimately meets, weaving up against a trunk several miles tall. Its leaves shimmer brilliantly, primarily in silvery blues and greens. The spirits of fae nest here, with newborns being produced from within the Great Tree before springing to life in the physical realm. The Angel Thediel theorised that fae often companion with humans with good intentions because of the conditions they're born under: "They know the lifestream and its complexities, its in their soul. And to know the lifestream is to know the heart of man."
Vanaheim, Realm of the Exalted
The realm closest to Yggdrasil, and also the smallest. Nigh-impenetrable silver walls surround it and the tree, and the city itself is something out of a dream, surpassing any man-made structure. An essence may be permitted to enter if they've accomplished great deeds within their lifetime. The creature entrusted with the governance of this realm is known as the Overseer.
Joruheim, Realm of the Brave
On the outskirts of Vanaheim's great wall is Joruheim, a large realm made up of sixteen city-states with the Gods of Uray as the acting idols. The essences of warriors that lived and died by a code often find themselves wandering here, with Joruheim's occupants mostly being made up of Wardens; the rangers of the Spirit World, tasked with protecting the borders of the other realms and enforcing the laws, as well as aiding in the protection of Vanaheim if need be.
Helheim, Realm of the Lost
An essence burdened by its actions in life will often unknowingly wander toward the borders of Helheim. The edges of the realm are bare and dry, the spiralling streams of life energy found within the Spirit World almost non-existent here. The deeper one ventures the colder it becomes, and the less likely they are to turn back as their guilt seems to entice them to keep moving.
The largest realm in the Spirit World is governed by the the Nethradin, demonic spirits that often torment the unfortunate. If an essence is able to overcome these troublesome creatures, they may realize the courage to escape Helheim. The Nethradin themselves were once the same burdened spirits that failed to escape and eventually lost the memories of who they were, changing their form.
The Lord of Spirits, Ryujin, chose a Nethradin to govern the rest, granting them the title of Shaytan. Since then however the title has switched hands several times, either through force or a successor. Even Ryujin is wary of the realm and unable to contain it, leaving them to their own devices and entrusting the other realms to remain vigilant. As of 1029AC, the Shaytan in power is Daeva the Infallible, successor to Aeshma of Sin in 925AC. Daeva has been exceptionally bold compared to his predecessors, going as far as to raid and conquer parts of Alfheim and Ifraheim, expanding the desolate borders of his domain.
Beneath the Shaytan the Marquis are bestowed land and resources. The Marquis are often as dangerous as the Shaytan themselves. Currently there are five, including a nameless hellhound that resides within the Shaytan's fortress. The hellhound is rumored to be Grigori; a powerful Nethradin that challenged Daeva directly and was bound to the form of a beast as punishment and a means of control.
The long river Styx crosses through most of Helheim. The faces of those desperate enough to contract with a demon can be seen beneath the waters, as well as the fallen essence's that chose to drown themselves in the river rather than seek redemption. The Marquis Sitri is a lizard-like creature that dwells within the Styx, his lands to either side of the river.
Alfheim, Realm of the Meek
Uraheim, Land of Water
Ifraheim, Land of Flame
The idol Ifrit is said to dwell within this realm. Their companion is a phoenix and they wield a whip that scorches the land with each strike. Ifrit is one of the most revered idols on the physical plane... notably, several tribes within Gehenna worship them, and those that do so are granted Flame of Ifrit, an undying fire that conjures itself upon the wielder's weapon and empowers their magic. Ifrit was said to have clashed and almost kill Aeshma of Sin on the outskirts of Ifraheim, leading to the latter stepping down as Shaytan of Helheim.
Terraheim, Land of Mountain
There are many different types of spirits that reside in the Spirit World, and these include:
An essence is a piece of a human's soul, a wandering spirit that's not yet ready to move on. They spend time in the Spirit World resolving their regrets, or return to the living realm. Some spirits will wander for thousands of years before rejoining their soul.
A creature that was either born in the Spirit World or outside of it, but has since grown to govern a portion of the realm. Idols often interact with the living realm and amass a following, empowering those that would follow them. The Gods of Uray are an example of this, believed to have once been a brotherhood of exalted warriors that discovered a new purpose while in the Spirit World.
After getting lost within the Realm of Helheim and becoming overwhelmed by the guilt of their life's actions, an essence will eventually change form and become a Nethradin, detaching from the memories it once obsessed over. The Nethradin's primary trait is that it can look into a person's sins and force them to confront them, often through taunting and violent means. While innately malicious, the Nethradin serves the purpose of reminding the worst of the spirits why they're here, and forces them to confront whether they can be redeemed or not. Very few are, with the majority succumbing to defeat and torture at the hands of the Nethradin until they eventually become them.
Handpicked by the Overseer, a Warden is in many ways a counterpart of the Nethradin, choosing to forsake all memories of its life in the living realm. Unlike the Nethradin, they do this completely of their own will, and the terms of clear: To become a hunter, exterminating Nethradin that venture past Helheim and any evil spirits who manage to escape into the living realm. Oftentimes spirit conjurers will seek out their help in banishing poltergeist. They're duty bound to destroy anything that shouldn't be alive.
A Warden have the powers to carry out their duty. Their eyes are two blue flames, known as "Death's Gaze", able to see a creature's lifespan, specifically whether they're dead or alive. Their soulbound weapon, an "Overseer's Blade", will instantly seal and bind the essence of the dead, making it extremely effective against their targets. They can also tell whether or not a person is lying, a passive ability where they're able to peek directly into a person's mind.
Wardens typically wear long dark grey robes, and a mask that covers their face. If unmasked, their face is completely blank, just skin. Their features appear over time to those that familiarize themselves with the Wardens.