Teleportation

Teleportation is among the most powerful effects magic can achieve. The ability to cover vast distances in an instant opens endless possibilities. It saves you a lot of travel time, and it makes travel safer. There's no fear of bandit or roaming monster attacks. Being able to instantly switch positions also confers vast advantages in combat situations. Got pinned down? Teleport away. Prefer ranged attacks? Teleport to high ground and let your attacks rain down. While teleportation per see is not a harmful type of magic, its applications can be very dangerous.
Therefore, the Gatekeepers keep a tight grip on the applications of teleportation magic, to make sure it is not misused. They prevent a lot of conflicts simply by devils and angels not being able to reach each other. Furthermore, they prevent said forces from swarming the Prime Material and turning it into a barren wasteland. Additionally, on the Prime Material they guarantee a certain balance of power by preventing entire armies from teleporting into unsuspecting cities and taking over.
It is important to keep in mind though, that not all teleportation magic is strongly regulated. There are two kinds of teleportation, which scholars coined minor and major.

Minor Teleportation

Minor teleportations are those where one changes position instantaneously without covering vast distances. Generally, every teleportation where you have to see where you're going can be considered minor. However, if you are standing on top of a mountain you can see pretty far. teleporting to the edge of the horizon would not be minor, even if you could see where you're going. Minor teleportations are limited to your immediate surroundings.

How it works

No one really knows. There are a few theories though. One proposes that these minor teleportations travel through the Ethereal Plane, where distances and time work differently than on the Prime Material. However, then why can no one remember traveling through the Ethereal Plane when using minor teleportations?
As all magic bends reality, one theory proposes that teleportation magic simply changes everyone's perception of the situation and thus makes it a reality. Basically by making everyone believe that you are actually standing somewhere else, you make it so. However, magic which protects the mind does not prevent these kinds of teleportations.

Major Teleportation

Major teleportations are those that allow you to travel vast distances in just an instant. From one side of the world to the other and beyond. These kinds of teleportations also allow you to visit the different planes of the multiverse. Using these will put you on the radar of Gatekeepers, who can exercise a certain amount of control over these long-range teleportations. This basically means, that without their express consent you are going nowhere.

How it works

Whereas the workings of minor teleportations are not yet fully understood, major teleportations have been explored in more depths. These types of teleportation make use of the Astral Sea, which connects everything together. It connects the different planes of the multiverse together, as well as different locations within the same plane. The Astral Plane does not adhere to the normal laws of time and space and is constantly in flux. At any time, there are certain points within the multiverse that are instantly connected via the Astral Plane. These points change constantly as does the Astral Plane itself. Major teleportation magic bends a small part of the Astral Plane to your will to give you, for just a moment, an instantaneous connection between two points.
The Gatekeepers can detect whenever someone tries to manipulate the flow of the Astral Plane. When someone does, they change the flow to put them in Anun'Deion, until a complete threat assessment can be done.

Elder Magic

As Elder Magic works completely differently to modern magic, so do their teleportation effects. So far the Gatekeepers have not yet managed to get a hold of Elder Magic teleportation. Therefore, they like to keep close tabs on anyone that still practices elder magic - such as Dragons and Giants. Over time, however, practitioners of Elder Magic have grown more and more cautious, and its practitioners seem to diminish constantly.

Portkeys

Portkeys are items infused with a part of the Astral Plane tethered to a specific location. The item can be anything, but usually, portkeys are small enough to be carried. They are activated by infusing them with a little bit of magic. Once activated, there is only a small window where one can grab the key to be brought back to the location the portkey is linked to. This is a one-way, one-use teleportation and can pierce through most teleportation preventing mechanisms.
However, creating a portkey is extremely difficult, and requires a very long time. High-level magic is required to manipulate the strands of the Astral Sea to tether them to a location and to the portkey itself. This process requires the expenditure of various rare materials (amongst them Dragon Blood) over the course of a few years. Therefore, Portkeys are extremely rare and only the wealthiest denizens can afford to make one.