Becoming a dungeon core

There will be mistakes as this is my first time writing a novel. Talk - " " Think - [ ] System - ( ) Author thought - [[ ]] Action - * * In a massive world, many times larger than Earth. Is filled with gods, magic, hundreds of intelligent species, dragons, orcs, goblins, and more monsters and beasts. Yet, they all point swords at each other. This world is made up of several massive continents. Most of these continents are on the brink of war or already in war. Yet some continents somehow find peace with each other. But other than that, this world is mostly stable. But that all changes when souls from different times, dimensions, galaxies, and universes, somehow end up in their world, either being summoned to stop some kind of demon king, reincarnated, or transmigrated. These souls change the balance of the world, either for the good or bad, always leaving a mark on history. Though these foreign souls that are not summoned are really rare, maybe 2 come every thousand years, as summoned ones are called by the gods and people during a time of need. But what would happen when a soul comes to this world, with a powerful ability, one that can change the entire balance of the world? Most would expect this type of soul to be a powerful dragon or a newly born god, yet most wouldn't expect the soul to be a small crystal rock the size of a fingernail. This soul will have a tough time trying to grow strong, especially since many more reincarnated, transmigrated, or summoned souls are popping out everywhere. With numerous foreign souls unintentionally traversing through the fissures of space and time into this new world, what prevents foreign entities from following suit? What safeguards are in place to prevent their invasion of the new world?