Orbis Alius (latin for the Other World) is the sibling plane to the Orbis Terrarum. It is sometimes known as the Otherworld or the Elsewhere. It has much higher concentration of mana if compared to the Orbis Terrarum, and all native fauna and flora are magical beings. Even mundane lifeforms that spread too far from the shores become tainted by mana given time.

Primal Magic

Despite being the sibling plane to the Orbis Terrarum, the Orbis Alius is incredibly alien to humankind. Everything is saturated with pure and primal magic, filling it with dangerous dreamscapes where the laws that govern reality are uninclined to behave as expected.   Staying too long in the Alius can be final for regular humans who wander into it's many shores. Even if they manage to escape the capriciousness of fae, the environment's magic seeps into the body, mind and soul, leading to disorientation, loss of time, paranoia, mania and even stranger effects brought about by the taint. Non-mages entering the Alius and not being rescued soon are most likely irreversibly mad, dead or lost forever. The initiated are safe from deadlier effects, but still susceptible to disorientation and loss of time, which makes it a perilous environment to traverse unassisted. It is advised to procure the favor of Fae guides or to bring artifacts that help mitigate it's effects.  

Spellcasting

Spellcasting in the Otherworld works a bit differently. Due to the magically saturated environment, mages can tap into the mana that permeates the plane to fuel their spells and rituals. Should they know how to manipulate it, which takes some practice, they can achieve more powerful effects than they would be able to in the Orbis Terrarum. However, this is effect is not entirely optional. If the spellcaster is not habituated to casting in the Orbis Alius, it is likely to lose control over it's own magic and cause accidents. Power and sheer will overcome technique easily in the Otherworld, and this effect is more suited to big bangs rather than rituals that require finesse.   This same effect, paired with the transient spatiality of the Alius, makes teleportation impossible to perform unassisted by Fae Gates.

Planescape

Domains

Since the Orbis Alius is nonspecular in relation to the Orbis Terrarum and also spatially volatile, it has been divided into Domains for ease of study and travel.   A Domain is a region of the Orbis Alius governed by a Dominus - an entity of extreme power that exerts dominance over an area. Many Domains have been charted - particularly with the help of diplomatic relations with the Fae - but exploring it's farther reaches is a very hard task and the plane has not been fully mapped. More than half of the charted expanse is part of the Tir na Nóg - the domains of the Fae Courts and clans to which most shores on Earth lead. Together with the crossings between them and known farther domains, the total mapped area almost reaches the calculated surface of the Earth. The actual size of the Orbis Alius is still unknown - and most explorers strongly believe it will remain so.  

Crossings

Wherever the influence of a Dominus wanes, the chaotic energies of the Orbis Alius are free to act on their own. These inhospitable regions, known as Crossings, cover large parts of the plane, and the effects of the plane's primal mana are even stronger in these places. Powerful beasts roam freely through vistas shaped by bursts of wild magic and vegetation - if at all able to thrive - grows in odd angles or to enormous proportions.   When two domains are too close together, sometimes crossings are affected by the clashing influence of the two magical signatures. This creates extremely unstable and inhospitable places.

Life

the Fae

Moreso, as it is not unlikely for certain fae species to wander into the Orbis Terrarum, encounters with fairies are known to have happened since before recorded history. Mages have succeeded in establishing relatively diplomatic relationships with the two Courts. While this warrants little in terms of safety and protection for travellers, it does allow for closer study of their culture and customs.   Rather than a single species, the Fae are plethora of different sentient species that identify themselves as a single peoples. Their society is a wild tapestry organized into different clans, families and tribes - some species exclusive, others not - which sometimes share culture and customs. Most respond to the authority of the Lords of the domain they inhabit, who preside over the the two great Courts. Others are considered rogues, and their treatment depends on their relation with said Lords.   True to the wild magic of their realm, regardless of species and, if any, affiliations, Fae are mischievous at best and often dangerous. Nonetheless, dealing with them is a necessary discomfort if one wishes to enter the Alius for whatever reason. As they are known to cross over to our plane, learning about them is also a matter of protection of the Veil.  

Beasts

The Alius is also home to several kinds of beasts, ranging from native species to descendants of species native to our plane that dispersed into the Otherworld and were molded by the taint over time. These beasts sometimes wander into our plane, but this is uncommon. Fae are as threatened by violent creatures as humans, and the more dangerous inhabitants are kept at bay, rarely reaching the shores. Because of that, those who cross over tend to be of little threat to humans. They do, however, present very clear threat to the Veil, as non-practitioners might find them and be inclined to interact rather than back off.   A clearer threat to both the humans and the Veil, however, are mundane animals affected by the taint. Eartlhy wildlife might wander into shores, or even into the Alius itself, and be changed by the primal magic of the Otherworld. Several known Beast species are Tainted Beasts such as Dire Animals and Spellbeasts.  

Others

Most known magical Beasts and Beings make their home in the Orbis Alius. The ones described above are merely the most commonly encountered, due to their proximity to the shores. It is known that some Dragons - once present on Earth, but now entirely extinct on our Plane - make their homes in the farthest reaches of the Otherworld. The occurrence of Elementals is also common on Crossings.