So, pretty simple. MC has been given a mission to save timelines of his Earth. Great events such as the assasination of the great Roman Emperor Julius Caesar suddenly fails.
Brutus saves him at the last second. How? Why? Pararell world? No, a new invasion of transmigrators from all over the multiverse!
They are there for one reason only. To get more powerful, to steal the essence of the world and to alter the timeline according to their whims!