Your magic-using or ex-Seeker character will need 5xp or more of appropriate and specific Background Skills to use the information in this section IC.
All Planes are infinite in size and any plane can therefore be reached from any other. No-one in the 1200s is actually sure of the exact way that the planes fit together, although many theories both general and specific abound within the circles of people who care about such things. Currently technology is not sufficiently advanced for these theories to be proven or disproved and so one mans guess is pretty much as good as another and the prevaling theory will be the one set out by the most powerful mage at any given moment:
Currently, Planar Theory holds that Urth and it's sky (the stars, moon, planets and sun) are the "Prime Material" plane - so named because it is logically assumed to be at the meeting point of all the others, being the most important. Almost all the sentient races of Urth have reached this conclusion independently although each holds that it is the most important because it is home to their own race. No-one knows or cares what beings from other planes might think about this, given that they are not even from the most important plane.
The various outer planes can be divided into four broad types - Pocket, Astral, Spiritual and Elemental. Across those definitions, planes can be termed "near" or "far", being a relative measure of how removed from the Prime Material they are in terms of accessibility. Note that these aren't measures of any kind of distance - for all the inhabitants of Urth know, the Paraelemental planes could be nearer to Urth than the Elemental ones, but as the Elemental Planes have a long and storied history of being contacted and travelled through, they are termed "near" while the Paraelements are "far". All planes are said to be surrounded by a "Veil", the invisible barrier that separates that plane from all the others. The Veil of each plane is different, and each Veil is differentially strong or weak depending on which other plane is being accessed by a being crossing it. This causes "routes" to appear, wherin it becomes easier to travel to, say, Arcadia via the Dreaming rather than straight to Arcadia because the Veils between the Prime Material and the Dreaming and the Dreaming and Arcadia are much weaker than the one directly between the Prime Material and Arcadia.
Veil strength can and does change, either by unknown natural process - the Veil between Urth and Arcadia has been weakening in the last few centuries - or by deliberate design such as the Ritual of Elemental Soul which created the Elemental Elves (which essentially punched great holes in the Veil between Urth and the six Elemental Planes). The Veil between a world and another world is, however, always the same strength when seen from either side (no one-way tickets) and always the same no matter where in those planes you are.
Planar travel is, now that the Elemental Lords have banned the practice of Gating, a tricky prospect as neither Spirit nor Magic use offers a way to the transfer of the caster. Mages summon things from other planes all the time - that's all a spell is, after all - and Gods send spirits to Urth from their home planes. The two easiest ways to achieve a little mind-broadening travel are to fall asleep - for the minds of sleeping sentients always go to the Dreaming no matter which plane their bodies are on - or to die, in the which case the Soul and Mind of the being travels through the Dreaming, into Limbo and then either into Oblivion or into a Spiritual Planar afterlife if your God will accept you. Going places while you're alive and awake, not to mention taking your body with you, is a little harder. Attempts to persuade demons, fae and so forth to carry poeple through their planes on demand in the manner in which elementals used to do it have not met with great success.
Mathmagics and Ritual Magic can both open actual holes in the Veil between worlds, creating a point - a "shallowing" - at which both planes merge together and an unwary individual walking through could end up on one or the other. Such things are rather unrefined, though, and most Planar travellers create more defined Portals that flip a being crossing the threshold into the other plane. Note that there's nothing to stop things on the other side coming the other way unless specific measures are taken in the Ritual or Mathmagical equation governing the portal.
Most Portals in TT's gameworld were created during the Age of High Magic, but some rare ones date back much, much further - the Dragon Lords built great Gateways that still survive. Note that it isn't just the inhabitants of Urth that can build them, either - from the Illithids (or even, some say, the Dragon Lords) to the G'lomtuu and the Beastmaster, beings from other planes have crossed over to Urth without waiting to be invited. Portals to Arcadia, the Plane of Shadow, Gehenna, various Pocket Realms, the Land of Fiction, the Plane of Song and others all exist frequently throughout Albion. In civilised areas they tend to be known of to the locals (at least in rumour) although there are always gateways which have not yet been found....or from which no-one falling through has yet returned.
There are still mysteries undiscovered in TT's multiverse, however - what lies beyond Origin and Oblivion is one (perhaps whole other multiverses), where the odd "parallel worlds" that sometimes appear come from, or what medium the beings said to have travelled through time have moved through. Time moves at the same rate on all the planes (though it should be noted that some Astral planes have a habit of distorting a person's perception of time, so it appears that they have been there more or less time than they have) - a person going to the Plane of Fire for a week and miraculously surviving will find a week has passed when they get back to the Prime Material.
"Metaplanes" are those worlds that appear to contain several of the other worlds - All the Spirit Realms appear to float "inside" the larger metaplane of Limbo, for example. If a traveller is forced out of a plane for whatever reason, that traveller will usually (but not always) be dumped into the Metaplane corresponding to the type of plane they were in. The destruction of a Plane is a rare event, but has been known to happen - Spiritual Planes are destroyed if the God governing them dies, for example - and dumps those inside if it does not kill them outright. Despite there being four types of plane identified by scholars (five including Metaplanes), there are only three Metaplanes - Limbo, the Dreaming and the Prime Material itself.
LIMBO is the Metaplane most commonly reached, as it lies "around" Pocket Realms and the Spiritual Planes - both of which are, in the experience of explorers, the most likely planes to eject the unwary out or be destroyed. Limbo is made up of raw, unformed energy - swirling semisolid ephemera that varies randomly in texture, temperature or opacity. Limbo has one major axis of direction - "Down" or "Up". The further a traveller moves "Up", the more energetic the Limbo matter becomes, but the Limbo matter - and anything that is in Limbo, including the traveller - always moves "Down" in a strong current. The current can be fought against, but not forever, and travellers stuck here will fall to Oblivion very quickly. At the "top", the limbo matter becomes so energetic as to be a light so bright it can't be looked at directly and is named "Origin". At the "bottom" is a black void named "Oblivion". Because the experience of floating in Limbo and looking toward Origin is best described as lying underwater at the bottom of a well, Limbo is also commonly called "The Well of Souls". Because of it's unformed nature, it is also commonly nicknamed "The Vortex", "Chaos" or "The Void". In the North it is rumoured that the Vortex can be worshipped and power drawn from it in the same way as from the Gods that the metaplane contains, but such power would be unpredictable and no one in Albion has followed that path to prove it for fear of what the consequences might be. One further danger to travellers - no one has ever reached it and returned, but by common agreement of the few planar scholars that exist on Urth, the Abyss from which Demons come lies somewhere beyond Limbo. If you meet another being on the road, be very careful to check who and what it is.
The PRIME MATERIAL lies "around" the Elemental Planes and is amply described elsewhere. There are stars and stuff in it, you can see them. Whether these stars are torches in towns in a land in the sky, light elementals sent to walk the heavens for eternity to guide the sun back each morning or really big alchemical reactions depends on who you ask, but it really makes very little difference to everyday life.
The DREAMING lies "around" the Astral Planes, and is entirely reactive - the experience of a being in the Dreaming is entirely generated by that being's mind, as the Dreaming forms itself into tailor-made worlds around the traveller. The Dreaming's planes are much easier to reach than, say, the Spirit Planes are from Limbo, and Shallowings form around a traveller that concentrates on a particular destination. The three Metaplanes are unusually well connected to one another. Beings travel to the Dreaming every night without realising it, and Shallowings between Limbo and the Dreaming abound. The Plane of Luca intersects with the Dreaming through shallowings in her role as Goddess of Nightmares.
The Six Elemental Planes of Air, Darkness, Light, Fire, Earth and Water are all well documented, and now that the Elemental Lords and Titans have banned access to their Planes by the inhabitants of Urth, universally deadly. Although mages can spin off an imperceptable fraction of their powers, the Lords of the Elemental Planes are alien and uncaring, seeing the Prime Material only as something to fight over.
Of the Paraelemental Planes, two are especially well known and four more have been encountered (or creatures from them have been encountered) in living memory. Gehenna is the Plane of Darkness and Fire, and less than a decade ago was the home-in-exile of an Elemental Titan of Fire that sought to usurp Kakatal's throne. Since that being's destruction, Gehenna has been invaded by Demons (who apparently find the pools of shadow and the fire raining down from the sky quite homey) and is hostile to explorers from the Prime Material. Shadow is the odd Plane of Light and Darkness, with numerous Shallowings to the Dreaming and the God-Plane of Luca. Shadow has a varied ecosystem of bizarre creatures (many of the predators are invisible) and is used as a shortcut by Elementals from the planes of Light and Darkness who are attacking the other side. The Plane of Smoke (Air and Fire) was until recently the home of the Human mage Maravannus, who used the Plane to build his "Quasit" servitors. Aside from those three, Paraelementals of Magma (Fire/Earth?), Ooze (Water/Earth?) and Ice (Water/Darkness?) have been sighted in the unclaimed lands around the Palatinate, leading some to suspect that undiscovered portals to those realms lie somewhere out in the wilderness.
The Quasielemental Planes are far less known, and the veil between them and the Prime Material nearly insurmountable. The inhabitants of the Plane of Light, Darkness and Water (who called themselves, or possibly their plane, or maybe both, "G'Lomtuu") started staging raids on the Prime Material for their own inscrutable reasons in 1282 DR and just as mysteriously stopped less than a year later. The Plane of the Daedra (who are apparently a race of Demons that had come to inhabit that realm) was emptied by the Dragon Lords just before their death, and the entire population vanished without a trace.
One other Spirit Plane exists, not that anyone has ever managed to reach it - somewhere deep in Limbo lies the Abyss, home of the Demons. While Demonic forces have spread into several other planes - including Gehenna and parts of the Dreaming - and "gone native" there, forming entire new races, the Abyss houses those Demons that remain in their "natural" state. It is possible to summon demons using High Ritual Magic although when they arrive you'd better have a pretty good reason for summoning them and be aware that where one has gone unwillingly, others may follow of their own accord. A summoned demon will eventually be drawn back to its own plane but to make sure it is really gone and not just hiding further ritual should be performed by the summoner.