Head-canon: Mordekaiser and the Metal
Mordekaiser's passive and abilities are based off of Metal songs. All of these songs relate to his character when he was alive.
Iron Man: This song is about a man who finds out about the apocalypse which would occur in the future through time travel. The man returns to his own time, but becomes a metallic man on his odyssey back to his own time. He tries to warn people of the impending disaster, but no one listens to him, inciting the time-traveler's rage. In his fury, the "Iron" Man would ultimately cause the disaster which he originally envisioned. Like the Iron Man, Mordekaiser may have been a tragic hero who was predestined to fall to the darkness in his heart, becoming the Iron Revenant that we know today.
Ace of Spades: A song which is about a high risk gambler. Winning and losing is all the same to this man, he still gets his thrills. Morde in his life enjoyed being reckless in life and undeath.
Creeping Death/Harvester of Sorrow: The first song relates to the Angel of Death from the Bible's Old Testament, the Book of Exodus. The Angel was the seventh and final plague on Egypt, and its duty was to reap all the lives of the first-born within the country. Correlating the Creeping Death with Mordekaiser, he's got serious megalomania. He would possess a strong belief that he's a divine tool, the Silver Scourge of Suffering. Another thing worth noting is that first-born has a second meaning, which is most important. In this game, who would be the most important player to their team? The carry. This song also fits the nature of Mordekaiser's ult. This ability can steal the enemy team's first-born, the carry, and use their soul against their own allies.
Moreover, my interpretation of the second song is that it is about a person who feels victimized, and betrayed. In Mordekaiser's story, this may hint at how Mordekaiser was killed the first time. The Master of Metal may have been betrayed, leaving him to die with no allies left, aside from his necromancers. One of the most poignant lines from Mordekaiser's current voice lines is "Misery loves company." He may enjoy bashing people with his powers and using their souls as puppets, but at his core, Mordekaiser is pure anger and suffering.
**Symphony of Destruction: ** This song is about how power can turn an ordinary person into a tyrant. There's snippets of the Pied Piper, and a war in the heaven. This relates to Mordekaiser's mortal life, his manipulative nature, and his ult.
Children of the Grave: This anti-war song's focus is about children initiating pacifist revolutions to ensure that they will have a better future. I find it ironic that this song was used as the name for Mordekaiser's ult because what his motivations are far from peaceful. Mordekaiser desires to conquer the rest of the world through the uprising of the Undead during the Harrowings. His "children" are his legions of Undead subjects from the Shadow Isles.