Early game

In the early game as morde you want to harass your matchup out of lane
if you are playing against hard cc you probably want to focus on csing unless jungle comes to help you. In the early game your passive gives you a ton of wiggle room to destroy your opponent in lane in combination with your W. Be careful as morde is more of a burst champ and has pretty long cooldowns in the early game

Mid Game

Post level 6 it is probably a good idea to roam and get kills with your ultimate.
You have a guaranteed kill if jungle comes to invade top or mid if they are not fed because of your ult and in some cases a double kill if the midlaner is low or is behind.

Late game

Mordekaiser has great late game scaling because of his E passive. This magic pen allows you to focus on building defensive or more offensive rather than being forced into a morellonomicon or a void staff for the magic pen. Save your ult in the late game for teamfights and try to pick out priority targets like the adc.