Mordekaiser is nothing without his passive.
His passive is strong but not broken. It only deals max HP damage and gives slight MS, but in return it is hard to proc.
His Q deals 0 damage late game and it has stupid long CD at all stages of the game.
His W is fair enough. Shield + Heal like Tahm Kench.
His E is a shit version of Darius E. At least Darius E pulls people hit. Morde’s E does whatever it feels like.
His ult can be countered with a 1300g item QSS and since Morde is a magic damage dealer, it is even more worth it.
If Morde didn’t have his passive, he would be a laughing joke of a champion.