Portals are permanent teleporation effects that link many places in and outside the world, including other planes. The devices riddle the Prime Plane, numbering in the thousands easily, though none has ever been able to determine the exact number of active portals.
An archway, that is a closed shape large enough to walk through, of some kind usually identifies the location of a portal, so those who cannot detect magical auras do not stumble into them accidentally. The portal itself is completely intangible. A portal is destroyed if the archway is also.
Many Portals also require portal keys to open; a key is usually a physical object, but it can also be an action or a state of being.
Naturally occurring portals will often appear at random; some portals only exist for a brief period of time, or shift from one location to another.
Let me tackle your second question first. Most of these gates tend to be controlled by the wizard, or small cabal of wizards, or priesthood, or even wealthy merchant, who stumbled across them -- or learned of them from someone who did (whereupon they usually killed the “someone” so as to make the gates ‘their’ secret alone). Individual Red Wizards tend to behave like everyone else when they learn of a gate: if they think other Red Wizards haven’t observed them, they try to keep it ‘their own’ secret. Ditto the Zhents, unless it’s a Zhentilar warrior who wants nothing to do with gates; such a one will report his find to a wizard but profess ignorance of the EXACT location of the gate (so he won’t be ordered to be the guinea pig who demonstrates it to a watching mage). Larloch has learned where a lot of gates are by magically farscrying unwitting beings, over the years, but even his minions rarely use them (except to pounce on emerging users they’re interested in). Halaster traditionally hasn’t been interested in gates that don’t impinge on Undermountain, but it remains to be seen what new attitude he may or may not have acquired towards such matters since the events of ELMINSTER IN HELL. Lone wizards and lone merchants tend to use gates they find in very similar ways: as ‘secret getaway’ routes, and as a way of reaching a new territory to explore (typically they’ll establish a hidehold or hidden cache at ‘the far end’ of the gate, to use as a second base; if things ever get ‘too hot’ for them at one end of the link, they relocate to the other). Merchant cabals and priests usually establish regular ‘trade-runs’ through their gates, and cabals of wizards tend to use them either to raid through, or (as merchant cabals often do) steal goods in one place and whisk them away to another -- often stealing items that are scarce at one end of the gate so they can be sold for great amounts at the other end. Gates whose ends are in different climes, of course, allow merchants to make ‘easy killings’ by taking fresh fruits to snow-choked winter locales, to sell for stiff prices, or to transport skins of drinking water into dry regions for the same purpose. As I’ve said many a time before, the Realms is riddled with hundreds of ancient gate-links, most of them long forgotten. Here are four sample gates:
Tesper’s Stride: Noted in some old diaries at Candlekeep as being the secret behind the financial success of the long-ago Waterdhavian merchant Ultath Tesper (no relation to the famous mage Tesper), this gate links a knoll near Amphail with a cellar somewhere under a Southbank building in western Scornubel (in what was once known as Zirta). Its active area seems to be two men wide and taller than a man carrying a boar-spear at shoulder rest (in other words, at least twelve feet high), and traversing it reportedly takes but an instant “of extreme cold, while falling through blue-white mists.” The northern end of the link functions ‘towards Zirta’ only when bathed in moonlight, but works ‘from Zirta’ at all times. It’s entered by mounting the westernmost of two like-sized boulders at the north end of a knoll that lies about a bowshot west of the main trade-road. Face south when atop the boulder, and step at least a handspan up off the rock. These boulders are about as tall as a cottage and as long as three horses, and have bird-dung-spattered but otherwise bare, smooth stone tops, but sides thick with green moss. Seek a knoll that has three bare ‘fallaway’ rock outcrops facing the road (on its eastern side), south of a tiny creek that comes out of a thick stand of blueleaf trees. Note that the scrub-covered ridge immediately to the north overlooks the boulders, and a watcher there has clear bowshot down at anyone arriving through the gate. The southern end of the link seems to function in both directions all the time. Taking it ‘north’ to Amphail is a matter of entering the westernmost end chamber of a low-vaulted, stone-lined cellar, going to its northwestern corner, and touching a particular wall-stone that’s visibly darker than the rest. The cellar is one of many in Zirta, where goods were and are kept safe from theft in deep cellars -- but just which building it underlies is uncertain, because this cellar was long ago linked to many others, and fitted with traps and access-chutes from above by the now-extinct Three Hands Coster (a local trading group of unsavoury reputation). At least two local thieving bands, the Blue Cloak and Rassavar’s Blade, keep watch over who uses the cellars -- or at least, who ventures west from the busy, used-by-many-caravan-companies eastern end of the cellars (which are rented out by a sinister fat, bearded man hight Ravosz Orthroth. Tesper used this gate to shuttle stolen goods out of Waterdeep, and to bring drugs, poisons, and perfumes from Scornubel into the City of Splendors, hiding them in chambers in the middle of ‘fixed’ casks of “winter ale” from Everlund, in the wagons of his passing-through-Amphail caravans. Some persons have definitely discovered and used this gate recently -- because fresh blood was found all over the north-end boulder last season, and a strange whisper-tale is making the rounds in Scornubel: of men ‘ridden’ by a chill presence in their minds, that ‘stole into them’ when they were using “a hidden, magical way.”
The Westwalk: this gate functions in one direction only: east to west, or outside to inside. It functions at all times, and is entered by finding a slight hollow about six wagonlengths south off the coast trade-road that runs east from the westernmost gate of the city of Westgate. This grassy depression is several hundred paces east of the gate, well within view of wall-sentinels, and they’ve learned about the gate’s presence (but not yet how to make it function), and now cut all brush in the area, to keep activities there in clear view. Only one being at a time can use this gate, though they may carry anything they can lift and move on their backs. There are three elements to the gate’s functioning: -- the user must step into the right puddle or (when weather has been very dry) ground-hole: the northernmost of the three -- the user must be carrying four silver coins and no other silver of any sort -- the user must utter the word “Alamaraska” (this may be whispered) If all three conditions are met, the user is whisked into a stone cupola (roof-chamber) atop “Mother” Mustivvur’s rooming-house, just inside the western run of the Westgate city wall. The cupola has window-openings on its east and south, a stone hatch in its (yes, stone) floor that’s usually barred from below, and a door in its east wall that opens onto an outside stair. Two guards armed with loaded crossbows guard the cupola at all times, and a third man collects the gate toll: the four silver coins. Those desiring to dispute payment are advised to beware his sharpened, poison-tipped fingernails (he’s built up an immunity to the unidentified venom he uses). This gate is, of course, used by persons desiring to enter Westgate unidentified or uninspected by the authorities.
The Lion Gate: this two-way, always-functioning gate has made many a Amnian fortune over the years, and its use is now monitored and taxed by the Athkatlan authorities (a guard of one concealed hired wizard, one armed official, and two guards armed hand crossbows that fire darts that have been sleep-venomed, who collect a toll of 5 gp per use, and detain and question any armed groups of users of more than four in number) at the city end of the link. That terminus is a second-floor room (now permanently occupied by city authorities) in the Sleeping Lion salon in southern Athkatla. One of several salons in the city, the Sleeping Lion is a place for discussions, dice and card games and the gambling that accompanies them, and light drinking and dining (of grapes, chopped fruit, nuts, wedges of cheese, and small spiced pastries). It’s the ‘lowest class’ (most tolerant and least opulent) of Athkatla’s salons, and has no guards barring admittance to persons purely on the basis of their apparent wealth or status, as the other salons do (the Lion does, however, have swift-acting bouncers to deal with violent or snatch-thieving guests, including both strongmen and roof-spies armed with blowguns that fire sleep-envenomed darts). Those who keep order in Athkatla want to prevent invasion of their city by, say, the Zhents or the Red Wizards, or anyone practising slavery or hustling kidnap victims through the Lion Gate. Otherwise, they don’t care how it’s used, so long as the toll is paid (failure to pay results in imprisonment and confiscation of goods valued at twice the toll; repeat offenders face harsher penalties). The Lion Gate permits the passage of only one living being at a time (additional living creatures will be teleported to random locales elsewhere in the Realms, arriving safely but with no indication of where they are, and no direct way of return). Similarly, non-living material up to the mass and weight of the living user’s body can pass through if worn or carried by the user; all matter in excess of either the mass or the weight of the user will simply vanish, apparently disintegrated (some users know this, and have used the gate to ‘disappear’ corpses by loading themselves heavily with supplies, and then dragging a body behind them -- which the magic of the gate obligingly causes to vanish). In any case of ‘overage,’ material in direct contact with the user’s body is retained when cargo carried in a packsack, satchel, or bag disappears. The other end of the Lion Gate link lies in a cave in rolling, bandit- and leucrotta-infested hills just north of the Lethyrstream and east of the Eastingreach coastal wagon-trail, nigh the Forest of Lethyr south of the Great Dale, almost clear across ‘known’ Faerun. There are several such caves in an area of loose-scree-covered hills surrounding an old gravel-quarry, and they all seep water and are used as lairs by a succession of opportunistic prowling beasts. Some bandits apparently know of the gate and lie in wait for those traversing it. To enter the gate heading to Athkatla, a user must climb the sloping rocky back of the correct cave, passing through a strange feeble green luminescence that seems to ‘drink’ some magics but not others, unpredictably (affecting only magic items, or actively operating spells, rather than written or memorized spells), to reach a ledge, and there touch the proper two stones at the same time. Just which two jutting stones is a secret, but they’re close enough together that an average-sized elf, lying down, can touch one with a leg and another with a hand. Regardless of how a user’s body is arranged, they reach Athkatla standing upright -- and also come from Athkatla standing upright, too (there’s enough headroom on the ledge to do so, even for a troll or ogre).
Jusk’s Stroll: This gate-link works in both directions, but functions only when the end it’s being entered through is in darkness (both daylight and lamplight temporarily cause it to ‘sleep’). It links a certain spot along the south wall of a dockside warehouse in Suzail with the alleyway behind (directly south of) the Dragon’s Jaws tavern (feature 39 on the 2nd Edition Realms boxed set “Grand Tour” book [page 54] map). The warehouse is Flar Oldbottle’s Nets, Ropes, Cables, Hawsers, and Cords establishment (a stout stone two-story building topped by a hammerbeam wooden loft and slate roof). It’s the second long rectangular building on the east side of the channel leaning to The Basin (feature 16 on the map), counting northwards. In other words, it’s the rectangular building that has a tiny shed attached to it on its north side (“Old Rak” Jumble’s Fresh Fish, a reeking sty of a place best avoided). The gate is located between Oldbottle’s and the warehouse of similar construction to the south of it (Red Sunsets Trading Company, a Suzailan merchants’ collective rental space for ‘small cargoes’), a little east of midway along the wall of Oldbottle’s, and in the “towards the Jaws” direction only works when the warehouse wall is touched in just the right spot (about three feet up off the ground, and near a waterstain that runs diagonally down the wall). The terminus behind the tavern is marked by a square of paving stones in the dirt, kept clear by city authorities. This gate is well-known to them, and they watch both ends of it to see who uses it. It’s also an open secret among dockworkers and merchants having business on the docks; other folk of Suzail know of it but not precisely where it is or how it works. Imprur Jusk was a long-ago wealthy halfling investor of Suzail. He discovered the gate and had no idea who created it or why, but he controlled the dock end of it until his death, of a winter fever (Oldbottle’s was once one of his warehouses). The link got its name from his nightly use of it to swiftly reach the upper-class taverns and festhalls he favoured, along the Promenade. It was said you could tell where Jusk had fared on many an evening by the strong smell of harbour fish clinging to this or that highcoin girl.
A series gate is a string of magical portals that connect two or more destinations through a series of intermediate locales. One example of such is the Lost Princess Road, which links the Purple Hills of Tethyr with such diverse locales as northeastern Amn, the mouth of the River Chionthar, the Bridge of Fallen Men, Silavene’s festhall in Waterdeep, and the confluence of the River Delimbiyr and the Unicorn Run. Another series gate is used as a the slaving route between the drow city of Karsoluthiyl in the Underdark off the coast of Baldur’s Gate and the drow outpost of Kyorlamshin in the depths of Undermountain. Many other examples of series gates may be found in Secrets of the Magister.
Although it serves much the same function as a series gate, the Trail of Mists is actually a series of fixed paths through the Border Ethereal. One can only step onto the Trail of Mists at certain fixed points across Faerûn, known as nexus points. A would-be "mistwalker" must know the precise location of the nexus point, as both magic and psionics have proven ineffective in pinpointing them, and move some part of his physical form through it. To access the Trail of Mists, the nexus point employed must also be wholly cloaked in some type of three-dimensional magical illusion, such as that created by a phantasmal force or improved phantasmal force spell. The exact nature of the illusion created is irrelevant, although most spellcasters create the illusion of some type of portal. Finally, a would-be mistwalker must be carrying a specific type of gemstone, and the exact type required varies from nexus point to nexus point.
Once on the Trail of Mists, a mistwalker can travel along one of the trails of thick mist linked to that nexus point until he reaches the next nexus point. All nexus points have at least one trail leading away through the Border Ethereal, and some have as many as a dozen alternatives. Upon reaching a nexus point, a mistwalker is instantly shifted back to the Prime Material Plane, and he must reenter the nexus point normally, subject to all the conditions listed above, to resume movement along the Trail of Mists.
While walking along the Trail of Mists, a mistwalker can see objects in the Prime Material Plane as if through a thick mist. The Trail of Mists never seems to pass through physical objects on the Prime Material Plane or rise off the ground. The Trail of Mists simply reroutes itself (and anything on it) around any new physical impediments from the Prime Material Plane that are moved in its apparent path. It is not possible to move even one step backward along the Trail of Mists or step off it in any way, nor is it possible to interact in any way with the neighboring Border Ethereal. Likewise, creatures of the Ethereal Plane cannot interact with mistwalkers in any way, although both can see and hear each other if in close physical proximity. If two parties are moving in opposite directions along the same branch of the Trail of Mists, the branch automatically forks and then rejoins so that the two groups appear to pass each other at a distance of 20 feet or so. Mistwalkers can cover as many miles (relative to the Prime Material Plane) per hour as they have points of Intelligence, although it is possible to move more slowly if desired. Groups that wish to stay together cannot move faster than the slowest being among them, as it is not possible to carry or pull someone along the Trail of Mists.