The Story of Mordekaiser
Mordekaiser was a warlord in the age before Noxus — when the region that would become the empire was contested between tribal confederations, when power was held by those who could hold it through force and through the terror their name produced. He was the most feared of these warlords, not merely because of his combat ability but because he had delved into death magic and discovered that the dead, properly compelled, were a resource that the living could not match.
He was killed eventually — it took significant effort and the combined force of the people he had enslaved and their allies — and his death was supposed to be permanent. It was not. He exists in a realm between life and death that he has partially claimed as his own, the Iron Realm, and he continues to extend his dominion over it with the patience of something that is no longer constrained by mortality. He will return to Runeterra. He is waiting for the right moment, which he has the luxury of waiting for because he cannot die.
What he wants is the full subjugation of both worlds — the living and the dead — under his dominion. Not out of cruelty, exactly, but out of the particular conviction of a man who has been the most powerful thing in every room he has entered for his entire existence: that everything should be organized under the intelligence capable of organizing it. He is that intelligence. The organizational structure is his rulership. He sees no reason to limit this to one plane of existence.