The multiverse can best be described as a series of bubbles of reality. Each bubble is a universe unto itself. These universes all have their own laws of physics which are roughly similar but often feature distinct differences. Some of these universes are “near” each other. The rules of measuring their relations do not follow Euclidian geometry, but they are consistent and with the right equipment can be measured. The area between them is called “The Space Between Spaces”, or sometimes just “The Void”.
Sometimes, two worlds will be pressed closely enough together that pathways between the two naturally open. Sometimes, these might be permanent portals, other times they are temporary rifts that can be learned to be predicted or even forced. The people of these worlds often become used to the idea of travel between their worlds even if in some cases it is rare or restricted.
Other times, worlds might be close enough that they don’t naturally connect but can detect each other through extra-reality sensory techniques (whether technology or magic). If worlds know where each other are, they might be able to form a bridge between the two. This is a massive undertaking and isn’t always possible, but it can form an artificial version of the natural portals that connect closer worlds.
Another technique for these more distant worlds is to form a small pocket reality that contains a ship and hurl it across The Void. This is risky, but can allow worlds that are too distant from one another to gain contact. If a people gain enough experience with this technique, they may even start sending ships out without a destination simply to explore and map even more distant worlds than they can see from their own reality.
It is often assumed that The Space Between Spaces is empty. After all, it is outside reality and since things need reality to exist, it follows that a lack of reality will produce non-existence. However, this is a mistaken assumption. There exist creatures of non-existence.
These creatures are not bound by physics, time, or even geometry. These are all facets of reality and since they exist outside of reality they have no need for them. They can expand in infinite directions, or contract to a single point with little bother.
Most of these creatures are small. Almost inconsequential, but some are vast. So vast that their presence can overshadow entire worlds. They typically breed by simply breaking off a piece of themselves and sending it on a mission. Sometimes these pieces come back when they are done and are reabsorbed, often bringing back more power of presence to their parent creature than they had when they left. Other times, they simply get lost or refuse to return, creating the vast spread of smaller creatures in The Void.
Typically, these creatures have no interest in interacting with reality. To them, they are simply rocks to swim around in their vast sea of nothingness. However, a small number take an interest in reality. When they do so, they will position themselves so they intersect reality. Doing so will cause a sliver of themselves to appear within the world.
From the perspective of the native inhabitants, they see a creature that violates all laws of their existence. The goals of this creature are often far beyond their understanding because they simply don’t have a full perspective of what the creature is or what it wants to accomplish. Such a creature can sometimes be damaged, but never killed. Since it is only a small piece of the creature, even the strongest weapons of the world cannot cause that much harm. The only way to truly harm such a creature is with a weapon of unreality, but most worlds have none. Instead, they must fashion a weapon made out of the creature itself.
Banishment is sometimes possible. After all, the creature is not naturally a part of any existence so to banish them from reality is a simple matter of getting the portion in reality to reposition to the rest of the creature. However, this does not kill the creature or even cause any harm to it. Whatever method it used to enter reality in the first place can be repeated. In some cases, this is some mortal acolyte that creates a summoning ritual to tear a hole in the fabric of existence and open a door. However, some creatures can simply enter reality at will. Banishing them will just cause them to immediately reenter. Possibly annoyed, possibly angry, possibly not even caring about the simple creatures who fear it so.