In this fantasy world, a great creator made twenty sentient species and gave them the world to live in, a few centuries later all the species are involved in a war and at the brink of mutual extinction. After the creator saw that his creations could not coexist in an open space where borders and resources were a reason to fight over, and so he forced all the remaining survivors into a tower divided into 100 floors giving five to each of the twenty species, as another way to try and keep them alive he made twenty kings.
The kings were made based on a blood pact between the creator and all the other kings, with that they could understand each other and became immortal against all other creatures of creation apart from another king so that if ever a king tried to conquer more than what was given the other could fight back while protecting their people. But to maintain the survival of all species even if a king is killed his soul would come back into a new body but with a cleansed mindset.
This peacetime lasted a few millenniums but as time passed contact with the creator kept decreasing and only the Humans at the top of the tower could still talk to him. This made the other top species envious as they thought the creator would allow humans to take control over the tower and kill all the others. With this in mind the Elves, Dwarves, Orcs, and Dragonborn, the species that lived on the floors closest to the humans, decided to join forces and send the four kings to the 96th floor where the human king resided and kill him and all other humans before he could be reborn so they would share the tower and have the grace of the creator again. In an attempt to stop the rebirth process after the four kings killed the human, they used magic to see the soul and chain it, each pulling the chain to his side and rupturing the soul into five pieces. Each king kept a piece with them to protect and the fifth one escaped into the depths of the tower to wait for the rebirth process to begin.
Although the Human king was quick-witted, and he understood why the other four came to him and started casting a barrier on the stairs to the 97th floor while accepting that he would die to them, but he would protect the rest of his people. Using the magic, the creator shared with him the king combined magic from all the twenty species on the barrier so that just four kings could not break it, they would have to learn the magic from the other species or kill all of them so that their magic would vanish from the tower.
But having only a small part of his soul the human king was deemed too weak by the guardians of the creator to be the soul of a Human as they had too much magic potential and so their souls were larger than most other creatures. These guardians made sure that any monster that died in the tower would be given a new body that matched the size of their soul and right now the king’s soul was so weak it could only be given to the body of a slime the weakest type of monster in the tower and now he needs to make his way back to the top to protect his people before the four kings break the barrier.