The World Tree is a cosmology option in Dungeons & Dragons, mostly associated with the Forgotten Realms campaign setting. In real-world mythology, the World Tree is known as Yggdrasil, and was integral to the multiverse of the Norse.
Planescape and Core D&D
In the 2E Planescape setting, Yggdrasil (along with Mount Olympus) existed alongside everything else and is an actual ash tree that was used as a means of multi-dimensional travel. It had its roots in Ysgard but grew onto and through other realms; allowing travelers to literally climb the tree and pass from world to world.
Since the Asgardian Pantheon also existed in 3rd edition, so did a separate variant of the World Tree cosmology, which probably mirrors the Norse version the most: The World Tree has its branches in Asgard, while its roots extend down into the three realms:
- Asgard: where the root of the tree touches the Well of Urd and is tended by the Norns of Fate. Asgard is home to the Aesir (Gods of War and Destiny) and the Vanir (Gods of Wisdom and Prosperity), it is also the location of Alfheim (for the Elves).
- Migard: where the root of the tree touches the Well of Mimir, the source of wisdom. Midgard consists of the mortal realms Mannheim (for humans), Jotunheim (for Frost Giants), Muspelheim (for Fire Giants), Nidavellir (for Dwarves) and Svartalfheim (for Dark Elves)
- Niflheim: where the root touches the river Hvergelmir which is the source of the world's creation. This is the land of Hel, and is where everyone who dies of sickness, old age, or accident go.
In 3rd edition the World Tree was used as a fundamental reworking of the Great Wheel cosmology where the planes do not share borders with one another. It was not an actual tree in the same way Planescape depicted, but rather the "tree" was created from a series of astral connections that linked some of the planes to a few others to create branches.
Changes from the Great Wheel
Unlike the Great Wheel cosmology, there is no attempt to balance the alignment of the World Tree model, nor are they defined by a set of moral philosophies; the planes are not necessarily situated next to their closely aligning neighbors, nor are they situated across from their diametric opposites if they have one. "Planes" exist more as clusters of deific realms rather than as self-sustaining dimensions and are organised more according to the deities which inhabit them; what links they have made with each other; and those links can forged and broken as the plot advances.
This has the advantage of making the cosmology quite fluid, where planes can move and shift as deities rise and fall, rather than being locked into an aligning location. Indeed, different depictions of the World Tree model have different numbers of planes on each side of the tree, and in different positions. In the World Tree Cosmology, gods tend to inhabit the realms of their sponsors or allies regardless of their own alignment and can create areas that would otherwise overlap and blend into each other in a Great Wheel and potentially create friction at the borders which cause portions to drop into one another like they do in other settings. Instead the planes are kept distinctly separate in the World Tree and are only connected through the various astral links.
- Arvandor: The home of the Seldarine, a scenic forest ported directly out of Arborea and expanded into a plane in its own right.
- Brightwater: A more civilised and refined version of Arvandor. Home to the deities of love and pleasure.
- Dwarfhome: Home of the Dwarven Pantheon (duh) called the Morndinsamman.
- Dweomerheart: The mountain home of Mystra. Her servants Azuth, Savras and Velsharoon make their own realms in the caverns beneath it.
- Gates of the Moon: The realm of Selûne is often said (but not always depicted) to occupy the highest branches of the World Tree and has links to every other celestial realm. Regardless of the astral connections, the Infinite Staircase has a conduit that connects this realm to everywhere else in the multiverse.
- Golden Hills: Home of the Gnome Pantheon; consists of seven rolling hills.
- Green Fields: Home of the Halfling Pantheon.
- House of Knowledge: Not actually a "house" but a campus of buildings in a forest. The home of the gods of inspiration and learning.
- House of the Triad: The mountain of Celestia directly ported into the Forgotten Realms setting unchanged, complete with the same angels and archons. The only difference is the addition of three lesser mountains circling it, providing the homes of Torm, Tyr and Ilmater (the "Triad") though other gods such as Helm make their home here too.
As with the Asgardian variant, the the physical "World Tree" also exists within the World Tree cosmology, making it doubly confusing when trying to describe to the unfamiliar where planes are located in relation to each other, especially when explaining that it is a world tree within a world tree. In this instance, the actual tree only connects the Celestial planes, setting its roots firmly in Arvandor and/or Dwarfhome, growing up through Dweomerheart, the House of the Triad and Green Fields with the tallest branches taking up the remainder of the planes. However, the World Tree is supposedly sentient and does not appreciate being used as a transitive plane and has Celestial Treants as guards blocking paths between realms unless travelers have good reason to use them.
- Abyss: Unchanged from the Planescape version. An infinite number of layers for infinite demons.
- Baator: Also unchanged, nine realms for the nine Arch-Devils
- Barrens of Doom and Despair: Vast expanses of black sand and jagged obsidian. Home to Bane and other deities of domination.
- Blood Rift: Depicted as the realm between Abyss and Baator, home of the Yugoloths and often gets missed from illustrations as to whether it counts as its own realm since no gods live here. It serves as the primary battlefield of the Blood War.
- Clangor: Home of the Goblin pantheon, a landscape of rusted war machines and abandoned weapons. Mildly positive dominant so soldiers resurrect each day to continue fighting.
- Deep Caverns: analogous to Pandemonium, tends to get missed from descriptions of the fiendish planes who often roll it into the Abyss. Home to the aberrant gods of the Illithids and Beholders as well as other alien deities.
- Demonweb Pits: Home of Lolth. Used to be part of the Abyss until the War of the Spider Queen when she dropped it out and made it into its own realm.
- Fated Depths: Deep oceans of crushing pressure and darkness. Home to the gods of the Sahuagin and Kuo-Toa. Like the Deep Caverns it often gets rolled into the Abyss, although it is not a chaotic realm in itself.
- Fury's Heart: A realm of storms, tsunamis and blizzards. Home to the deities of the destructive side of nature.
- Hammergrim: Home of the Duergar pantheon.
- Nishrek: Home of the Orc pantheon and is an endless field of trenches and fortifications. Like Clangor is also positive dominant for warriors to continue fighting each day.
- Supreme Throne: Home of Cyric and no other, thanks to his enormous ego.
Like the World Tree, the Fiendish planes have their own unique feature that links them together. The River of Blood acts much the same way as the River Styx does in Planescape, snaking its way through the evil planes and providing a means of relatively seamless transportation, even drinking its waters causes mortals to lose all of their memories in exactly the same way as the Styx does. The slight difference is that the River is said to have its origins in the Abyss (instead of Pandemonium) and its terminus in the Fated Depths (instead of Acheron) but it is otherwise the same. Only the Supreme Throne and the Demonweb Pits are untouched by the river, but they have narrative reasons for separating themselves from the rest.
- Dragon Eyrie: A single eternal mountain acts as the afterlife for Dragonkin and realm of their gods (including Tiamat, who otherwise resides in Baator). Dragons are still fiercely territorial and don't appreciate humanoids moving in to make settlements.
- Heliopolis: Realm of the Mulhorandi pantheon. Dragged along when they immigrated into Toril.
- House of Nature: A plane mostly unspoiled by civilisation where the petitioners turn into celestial animals over time, similar to the Beastlands. The Guardinals have their origins here instead of Elysium. Despite this it is not a "good" plane, though each deity can impose whatever conditions they like in their respective area.
- Jotunheim: Home of the Giant pantheon, a wildly mixed realm of varying terrains and environments.
- Warrior's Rest: Home to Tempus and the gods of battle. Acts a mix of Ysgard and Limbo in that the terrain shifts and changes all of the time, but can be altered and shaped through force of will. Though petitioners regenerate every night to continue battle there is no inherent positive trait allowing this and it can be revoked if Tempus wills it.
Fundamentally the same as the Planescape versions, the realms of Fire, Water, Air and Earth, as well as Positive and Negative energy are all represented unchanged.
- Astral Plane: While it is similar to the version of the Great Wheel, the fundamental difference in the World Tree version is that the outer planes do not overlap, so it is impossible to accidentally cross the boundaries from one plane into another (like falling from Arcadia to Mechanus for example) In fact, there is no single "Astral Plane" either; and travelers who occupy the link between two realms have absolutely no chance of meeting characters in a different conduit. Instead of the Astral plane linking all planes like they do in other settings, all realms connect to the Prime Material via Astral links instead.
- Ethereal Plane: Coterminous to the Prime Material, exactly the same in virtually all settings.
- Shadow Plane Also Coterminous to the Prime Material as well as all other worlds in D&D (such as Oerth) allowing for travel between campaign settings. The Plane of Shadow has the distinction of being the home to the gods of darkness like Shar and Mask, as well as being the origin point for certain villainous races such as the Shades.
- Cynosure: A plane that does not connect to the Prime Material at all but instead connects to the divine realms of all of the other gods. It is absolutely impossible for mortals to reach it without a divine invitation. Here magic is completed flattened, making it impossible for any single god to claim dominance. However the plane is completely immutable and serves no strategic purpose other than to exist as a meeting ground for deities to hold court or air grievances.
- Fugue Plane: another unique and important plane in the World Tree model. Instead of departed souls immediately dropping into planes of their respective alignment as they would in other settings, they temporarily stop off in the Fugue plane to await collection by agents of their patron deity. This means that atheism is a no-no in the Forgotten Realms, as godless souls get denied proper afterlives and have to sit around bored for eternity, make deals with Baatezu to escape, or get absorbed into the mortar of Kelemvor's city of the dead and vanish forever. None of which are very appealing choices.
- When a deity dies or disappears, so do all of the astral connections to its realm. While this gives the apparent appearance of it no longer existing, it is believed that those abandoned realms are instead drifting outside of the cosmology waiting to be rediscovered or reclaimed, such as Amaunator's realm of Eternal Sun or Bhaal's Throne of Blood.