I have posted a similar question where I asked how can two realities mix into one reality in which I received many helpful comments. I realise I should've expanded on the details I posted, so I will refer to points raised in the previous question and new points I had forgotten to include or came up with after reading the responses.
First off, I am an amateur dungeons and dragons player and aspiring novelist. I want to create a science-fiction fantasy which I can use as both a campaign and publish as a future novel. In the plot, I create a world in which two realities combine to form a new reality. One reality is a D&D reminiscent fantasy world, the other is a modern setting like our current 2020 (or is our current 2020, I haven't decided) and when they collide, this results in the world of magic mixing with the world of science. Maybe they mutate to form new species and technology. Currently, I plan on introducing a multiverse in which we can see cities which have dragons and skyscrapers living side by side. Where we see how the fantasy world impacts the socio-political context of the modern world and how technology can be aided in its development with magic. This is the era I will set my story in.
However, in order to have a sense of continuity and give my readers an explanation of how this all happened, I am developing a timeline in which the reason to why the realities collided is magic-based. I do want to go in-depth on the science side and how scientists would be able to detect the collision or phenomena related to the collision. I also want to explore how this could affect the rules of physics and the Exoplanets that could appear or disappear as a result of the collision because I am planning on eventually having my story explore magic aided space travel(kind of like Voltron I guess).
Now, the reason why the realities collided is a little experimental. There lived an academic who studied religion as their doctorate. Naturally, they had a friend who was an... active historian. One day the historian discovers an artefact unlike any other. A tome made of an unknown material, practically untouched by time. The historian, being close friends with the academic, brings it to them. As the two study this artefact, they find drawings of unknown gods and monsters, the culture and religion of which are unknown. (I have yet to decide how this book comes to being in the first place, maybe someone of their world slipped through the cracks of reality and made it back).
In the fantasy world, I have made it with many polytheistic religions. The one thing that ties them together is how each deity gains power. The only way for a deity to absorb enough power to use magic and exist as more than an immortal (which is how they are born when they are spat out into the world for whatever reason) is to amass a certain amount of worshippers. Deities need the power of a being's spiritual essence(be it their soul, their chi or as Star Wars puts it- their 'force') to use magic and aid their followers and strengthen their abilities. It is because of this that each deity can detect the people who follow them. They can detect where they are and how many followers they are as well as how much they believe in the deity.
So, when a being with a strange language from an unknown location begins to study a deity, it draws their attention. After first contact and the whole, this is where I come from montage (I haven't got the details yet), something happens to make the deity decide to willingly or unwillingly bridge the two realities together or combine them together.
As I said before I am focused on finding out how the collision or connection of two realities can affect one or the other and how this can be analysed and viewed by scientists. I will probably have to look into how the magical side monitors this new reality but that's for another day.
If anyone can give any ideas or some sources to study that would be great