Hey! It is me, the author, again! I was supposed to write the prologue of this fanfiction here, but it will just be the same as many other of the Tensura fanfictions, so I will just give a brief summary. This is definitely NOT because I am lazy! Don't misunderstand me!
Basically,
20 years have passed since the end of the Great Tenma War, and Tempest has become a superpower. Rimuru, even though he is Veldanava's successor and is unrivaled in the world, can't free himself from the paperwork he hates so much. One day, Ciel suggests they create a parallel timeline, and time travel to the time when Rimuru had just been reincarnated. It would be a parallel timeline, so the current timeline would continue to exist, and Rimuru would create a [Parallel Existence] of himself to be in the main timeline in his place. And so, after getting everything ready, Rimuru goes back in time to a parallel timeline to the time when he was reincarnated as a slime.