Five champions chosen by the summoners are sent to an unknown world, to locate and destroy the undead champions, who has managed to escape the spells the Summoners put onto them and has traveled to this unknown world to wreak havoc upon its citizens and create a new undead-world, where they shall grow stronger and more powerful, before they are ready to take over all of Runeterra.
(gore tag is only for mild gore and not until later chapters. This story will not be a gore-feast, so don't worry about that)