Played against a Mordekaiser today for the first time.
I was Pantheon top, knew the gist of his skills. Gets shield off minions, has a ranged AOE I want to avoid, so I played safe and poked with Q, except..I couldn't damage him. Not saying I attacked him and didn't do very much damage, I'm talking I'd throw my spear at him, no shield, and it literally did nothing to his health.
This happened multiple attacks including him taking a full duration Heart Seeker and his health not moving. Is there some kind of hidden passive to Mordekaiser I'm not aware of that blocks attacks or something?