I've got an entire universe, but for anything to make sense, I've got to start with a bit of quantum physics (don't worry if it's convoluted and doesn't make a lot of sense - you literally have to rewire your brain to understand quantum physics, and I've personally only every been able to get halfway there).
Every particle in the universe is stamped with something called "information." Information can be understood as the history of this particle (so, the particle was first part of a star, then a supernova, then soil, then a plant, etc.). In the universe of Re:Apotheosis, it can carry much more than that - if the particle comes from a neuron from somebody who is thinking about a story they have just seen or read, it can also embed the DNA of that story and its entire world (including the physics of it, which is important).
When enough people read or see a specific story, an "information stream" is created - think of this as a ribbon of information that turns the story into a real place in an alternate dimension, and maintains it in compliance with the story. The rules that govern the information stream (and the world that it creates) are imprinted in the particles that create it. So, the people reading or viewing the story have a tradition of stories in which somebody can move from one world to another, it becomes possible for a character to travel back up it to the world that created them.
Or, put another way, Edgar Rice Burroughs started the whole "transported to another world" at the start of the 20th century with his John Carter of Mars books, causing the information streams to become "leaky" and for fictional characters to start falling out of their story worlds into our real one.
So what happens when a character with the powers of a god travels up the information stream, sees the world that created them, and doesn't like what they see? That's the start of a war...