Teleportation is a very broad field of magic that deals with the power to travel instantaneously between different places. All kinds of teleportation require a great amount of magical lore and power. Teleportation magic, while producing similar effects, varies greatly in nature and origin, and can be used in the most various ways.
Portals are possibly the most widely used contraptions for teleportation. A portal, in magical terms, is described as a fixed device (or simply a place) that sends anyone who activates it to a certain destination. A portal can be activated by a mechanism or spell, or it can be permanently active. Some portals can change their destination, although this commonly requires a lot of skill and care. The convenience of most portals is their ease of use and maintainance. Once it has been built, it requires enormous amounts of energy, but can fare quite well on its own. Its user requires no more skill or knowledge to pass through it than for passing through a simple doorway.
Most portals are "linked". Two portals stand in different places, and anyone entering one portal will emerge from the other. This is very convenient for two-way journeys, but it also is a far more stable contraption than a one-way portal and requires less energy per use.
The portal to Avernum was such a linked portal for the latter reason; the one-way nature was achieved by a simple barrier. The mass teleporter built by the Empire was also linked, in order to achieve its unprecedented capability.
A one-way portal sends its user to a place where no other portal is installed. Although the effect is similar, this is far more complex magic and requires a lot more power. A one-way portal is necessary of course for destinations that have never been traveled to before (although a spell is used more frequently in this case), but it is also useful if the connection is not supposed to be detected on the other side, or the user is not supposed to return.
Mahdavi of the Tower of Magi built a portal that was capable of sending its user anywhere in Exile, given a set of coded coordinates. The Cult of the Sacred Item possessed a similar portal, although it was far less stable, more dangerous, and more difficult to configure - the cult itself suffered various fatalities through its use. The Empire has also been reported to utilize unstable cousins of the portal.
Variable portals are by definition one-way, although some linked portals are able to "switch" between several destinations (where other portals stand) in a way similar to a variable portal.
This portal originally used a coordinate system for destination positioning. The coordinates were three syllable phrases that were spoken loudly before entering the portal, which allows it to "lock" onto a destination. For example, Ell Pit Esk would calibrate the portal to a spot in the Black Waters. These coordinates were calculated by mages skilled in teleportation, and often recorded on obelisks scattered throughout Avernum.
Some places can form natural portals due to heavy magical residue or a natural source of magical energy. These portals are usually very unstable and risky to use.
A very complex field of teleportation magic is the analysis of a portal's behavior in order to gain various information. Seles, probably the Tower of Magi's most talented portal mage, had developed this skill to such an extent that she was able to detect any other source of teleportation magic in the vicinity of a certain portal. The residue of the teleportation affected nearby attuned objects, such that by observing a portal she was able to recognize the origin and destination of such a transport.
Through the analysis of a portal, it is easy for a trained mage to identify where it leads. It is also possible to detect the presence of a one-way portal's destination, from the heavy traces of residue it leaves behind there. Only very skilled mages are able to deduce the location of the starting point, however. The same applies to Variable Portals, but they must have been frequently used to travel to the given destination to leave a distinct mark at that place.
Technology of Portals
Humans have advanced far on the lore of creating portals, but it is believed this science originated with the Vahnatai. Even human-made portals are formed with the use of structured crystals, a magic known to be a specialty of the Vahnatai, suggesting that the lore may at one point have been learnt from them.
The Amulet of Returning was given to the adventurers, who saved the Empire, by destroying the Vahnatai Plagues. Anaximander gave them the amulet to return more quickly and report their actions against the monsters. It can be used only in a specific range around Fort Emergence. When they were near Gale or more to the northern edge of Valorim, they were not able to return through the amulet.
While most portal magic is highly specialized and requires years of study to master, some quick teleportation spells exist. There are three types of simple (relatively speaking) teleportation spells.
A powerful mage can place an anchor on a place and cast a spell that will return him/her and anyone or anything they are in contact with to the place of the anchor. This is the same magical concept that powered the Amulet of Returning that Anaximander gave the Surface Explorers. The stronger the mage is, the further away from the anchor the spell will work.
With the aid of a clairvoyant or scrying pool, a mage can choose a target, be it people or objects, and cast a teleportation spell to bring them to the mage. The stronger the mage, the larger the objects can be, and they can be brought over larger distances. Still, this technique requires at least one very strong mage or wizard. Notable examples of this include Seles and Prazac's wizards rescuing the Surface Explorers from the Tower of Magi disaster and Rentar-Ihrno's fortress, respectively.
Similar to retrieving, mages can either place an anchor in a room or place and send things from where they are to the anchor, or they can use a clairvoyant or scrying pool to find a place to send items or (rarely) people to the scryed location. Only the most powerful mages can transport people - most prefer to actually create portals as a safer alternative. The strongest mages can retrieve and transport people in a single act - rescuing them from danger and sending them somewhere else instantaneously.
Teleportation, be it by portal, by amulet or by spell, typically requires vast sources of power and great skill. In portals and amulets, the users themselves contribute little, but the caster of a teleportation spell has to be very powerful.
Some items are designed specifically to aid this process in the form of a Magical Catalyst either by lending additional power or by further focussing the power that is provided by other sources.
Portals require such augmenters practically by definition, which is why they are not called augmenters in this context. However, the power source of a portal - functioning like an augmenter - is exactly what allows it to be used by unskilled persons without any magical power. Small, short-distance portals need only a few small crystals that are usually set into the portal frame. Larger portals, however, are almost always seen in the vicinity of a few huge magically constructed crystal pillars. These pillars act as a power source, drawing arcane energy from the surrounding air and focusing it to power the portal.
In amulets, the power source is commonly included with the item itself - however, some amulets lack an augmenter, and are instead powered by a limited amount of energy imbued in it by its maker. These amulets can be used only very few times before they need to be recharged by an enchanter.
Carried by a spellcaster, the augmenter typically takes the form of a crystal or an amulet - in fact, such an augmentation amulet looks very similar to an amulet that actually contains a teleport spell. When a mage uses an amulet to teleport, it is not necessarily obvious whether the amulet works autonomously or simply augments the mage's spell.