Before, we had a hellacious warlord who used necromancy to become a lich of sorts, with his spirit residing in his armor, while his bones were the key to his downfall. He conquered. He was fucking unstoppable. It took someone (probably LeBlanc) stealing his skull and having it sealed away on the Blessed Isles to stop him. That's how epically powerful he was. He crushed mortals underfoot. He was undefeatable, and would have conquered everything if not for betrayal from within. And with the Isles blowing open with the Ruined King's actions, his skull was free to be reunited with his other bones, and reanimate his armor, allowing him to roam free again.
His new bio? It strips EVERY. SINGLE. BIT. of power out of him. He's not longer an accomplished necromancer. He's just some stupid, idiot barbarian wishing to go to the equivalent of the halls of Valhalla. When his religion turned out to be a dud and he was stuck in limbo, he managed to whisper to cultists to bind his soul to some metal armor so he could roam free again. He did, until more cultists severed those bonds, and now he's corrupting the souls of those he killed while he's in purgatory again.
All of the legendary nature that he once encompassed is lost entirely. His one weakness, his bones, are gone, his intellect as both a warlord AND a necromancer is gone. Now he's just some random, bloodthirsty guy who killed a lot of people. He's a diluted, watered down version of who Mordekaiser used to be, and his story lacks any kind of mythological impact his previous lore used to have.
This isn't really Mordekaiser to me anymore. He's just some meathead Viking who got pissed Valhalla wasn't real. He's not an unstoppable force like he used to be, he's not a world ending threat like he used to be. He's just another irritating blip on the radar of all of the supernatural entities causing havoc in the world.
And the biggest question of all is why? Why did he lore have to change? It was already sufficiently updated to make him an epic part of the mythology of the world. He was damn near perfect in how he fit with everything, and bridged areas like Noxus and the Shadow Isles. What was wrong with his previous lore that it necessitated such a massive overhaul that was clearly a downgrade?