Laws around Portal Operations

Since Aoskar, the God of Portals, was slain, many have explored the ways that portals can be created, used, and unmade in a variety of ways. While some spells can pull beings from other planes, these do not use formal portals and it's widely regarded that such spells can only be cast in energetically neutral places such as the Material Plane and parts of the Outlands that are not too close to an interplanar gate as the energy from other planes can cause adverse and unwanted side effects.

Natural Portals

Some portals appear without needing to be requested. The Planescape is a complex place with it's own rules (for instance, the Rule of Threes), so naturally portals can appear and disappear on their own accord leading to unknown locations. Interplanar Landmarks such as Yggdrasil, The World Tree and the The World Serpent Inn are places where natural portals can be found en masse. Some are constant, appearing in the same place and going to the same location, while others are random, with random locations at the landmark itself and going to random locations throughout Planescape. While Yggdrasil can be considered an interplanar train station thanks to the majority of its portals being fixed, the World Serpent Inn has only a handful of fixed and reliable portals. Some doors on other planes may always lead to the World Sperpent Inn, but even the Inn's front door doesn't always lead to the same place.

Portal Keys

Some portals always exist in a certain location, but require a "key" to be activated. The front door at your favorite ice cream store might secretly be a portal to a pleasant part of Bytopia if you have the correct key. Items, feelings, or actions related to a specific plane are all possible portal keys. Sample portal keys to Mechanus (the plane of regimented order) are a small gear of a particular metal, the copied text from the centermost page spread of a legal text, reciting prime numbers, and a focus-enhancing potion. For Limbo (the plane of creation, dissolution, and transition) portal keys can be a statuette of yourself that you carved and then broke, unbalanced die, or a fake word you are convinved you know the meaning of. While many things could qualify as portal keys, a specific key can only open one specific "door." The ice cream shop portal to Bytopia requires a maple seed. Any maple seed can open that portal, so anyone can have a Planescape adventure. But only at the ice cream shop can a maple seed get you to Bytopia. This means that there are plenty of unintentional journeys if you are not aware of what doorways require certain keys. In Sigil, this means a lot of first timers are warned to keep their pockets empty to avoid accidental separation from their tour group.

Wildwalkers

There are only two known species that are able to summon portals at will without using any keys or components. The first are Aoskian Hounds controlled by Hound Handlers of the Travel Guild. Their uncanny sense of smell and ability to create portals mean that people only tend to employ hounds if they are tracking down someone throughout the Planescape or if they require an unusual amount of anonymity. There are only a dozen working Aoskian Hounds and Handlers.
The second species that can travel the Planes at will are Axani Wildwalkers , very young female Axani children who between the ages of 3 and 10 can create portals at will. They are venerated by their people, and are also employed by the Travel Guild in Sigil.
When traveling with Wildwalkers, it's not so simple as stepping through a portal and immediately appearing at your destination. There is a short period of existing in the "space inbetween" worlds. Frequent planewalkers report the feeling of something hunting them down while they are traveling via these portals, and there are rumors that those who travel too much with Wildwalkers will eventually be found by this horrific entity and they will disappear from existence entirely.

Type

Metaphysical, Arcane