When a Portal is active, the surroundings light up brightly.
The kind of runic manipulation required to create Portals is not available to Shadowhunters, and so they need to be created and set up by warlocks. To get around this, a large number of permanent Portals have been established to transport Nephilim to and from Idris without having to hire warlocks for every trip, which is still the case in other parts of the world. Warlocks are now also able to easily create Portals.
Originally, Portals had to be closed manually by their creator once they were no longer needed. But in recent years, warlocks have been able to create Portals, the kind of Portal usually used today for safety, that can be sealed or close automatically after a certain amount of time has passed.
Additionally, a person needs to be familiar with the destination when going through a Portal. Otherwise, it will take the person to whatever place he/she is thinking of, or the place the last user of the Portal went to.
The Clave guards any magical entry into the city, and the demon towers ward against Portaling directly into Alicante unless the traveler is expected on the other side. However, it is possible to Portal to Idris, outside the city.
The only permanent Portal open in Alicante, for the use of the Clave in times of emergency, is in the Gard. Because of the demonic Marks on every Portal's frame, the Portal and the whole room containing the Portal is "reverse-warded", whereas it is blocked off as a place outside the protection of the wards, much like an Institute's Sanctuary. Because of the risk, the exact location of the Portal is a closely guarded secret.
The invention of the first successful Portal was a product of a collaboration between a Shadowhunter and a Downworlder, namely Henry Branwell from the London Institute, who drafted the plans for the device, and Magnus Bane, the warlock, whose name as the co-creator of the Portal is not recorded or credited in history, who used magic and demonic runes to make it work, in 1878.
There were two major requirements in creating a workable Portal for Shadowhunter use—keeping it stable and safely open for the necessary amount of time and safely closed when no longer needed, and accurately controlling the destination that a Portal would open onto. While the first of these was quite easily solved by Branwell, directing its destination proved to be a problem due to the limitations of the Gray Book Marks. The Portal could not be tested until it was fixed.
The solution was discovered by Henry and Magnus, who merged two runic systems and the magic inherent in the mind of the one traveling through the Portal. First, a "frame" of Marks (which have analogues in both seraphic and demonic runic systems) was created, and inscribed within and around this frame is a set of demonic runes that are drawn in an unstable, unfinished state. These runes, however, only specify the destination in vague terms. To "tune" the Portal to the exact location desired, the user of the Portal must picture clearly in their mind the destination they are traveling to. The Portal detects these details and modifies the demonic destination runes on the fly, to exactly describe the far end of the Portal.
Clary Fray first used a Portal at the Sanctuary, run by Madame Dorothea in her apartment, the same one used by Luke Garroway some time before. Since it was last used to go to Luke's house, when Clary and Jace leapt through it, it took them to the same place since Clary had no destination in mind when she passed through it.
A few days later at Renwick's, Valentine Morgenstern used the portal to go to the Wayland manor in Idris, where he and Jace had lived seven years prior. Once past the threshold of the frame, Valentine destroyed the Portal, shattering it into pieces. Jace kept a piece, hoping to be able to see his "father" through the broken shard.
The Lightwoods employed Magnus Bane to create a temporary Portal that would bring them to Idris. Left behind, a desperate Clary used her power to create runes and opened up her own portal. Because her arrival was unannounced, she was not able to Portal into the city. Instead, her Portal opened above Lake Lyn, which was supposedly as close as it could take her to city.
In December the same year, Julian Blackthorn contacted the Clave from the Los Angeles Institute asking for help during an attack by Sebastian Morgenstern and his Endarkened Shadowhunters. The Clave opened a Portal to Idris in the Institute's office, which the Blackthorns and Emma Carstairs were able to use to escape.
In 2010, Portals all over the world began malfunctioning. Numerous times when trying to open a Portal to and then within Shanghai, Magnus Bane found the Portal to open into a demon dimension, with such creatures spilling out. It was later discovered that this issue was caused by Shinyun Jung and Ragnor Fell's attempts to break the wards keeping Sammael from entering Earth and all the Portals were opening into his new realm, Diyu. The pair even attempted to open a permanent Portal to Diyu in the Shanghai Shadow Market but it was immediately closed by Magnus. They later used a Portal to escape Diyu and then return home, which, by that time, Portals were working correctly again.
- In the film The Mortal Instruments: City of Bones, there is a Portal in the New York Institute. Jace explained that using a portal requires extensive training. Valentine, with the help of Hodge Starkweather, then used this portal to enter the Institute, and is later also used to send him away. The Portal is also substantially different, made of a liquid substance that can be held and moved; it can even be frozen and form snow.
- In the TV series Shadowhunters: The Mortal Instruments, there is a secret Portal in the forest guarded by Seelies because the intersection of an alternate dimension intersects with their realm. This Portal can be used to travel to a specific other dimension. In this other dimension is a Portal—the only Portal left standing in that world—that functions more like Portals in the book; that is, it can take the user to anyone he/she simply thinks about.
- In both adaptations, the misuse of the Portal, specifically not being able to properly visualize the destination, not having a specific destination in mind, or not knowing the destination the open Portal leads to before entering the Portal can cause the person to get stuck in limbo.
- Lady Midnight
- The Shadowhunter's Codex
- A Long Conversation, a Lady Midnight extra
- City of Glass
- City of Bones
- Clockwork Princess
- City of Heavenly Fire
- The Lost Book of the White
- The Mortal Instruments: City of Bones
- Shadowhunters 1x10: This World Inverted
- Shadowhunters 1x02: The Descent Into Hell Isn't Easy
- Shadowhunters 1x13: Morning Star