Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This is a DPS Mordekaiser.
First of all, I start with a Doran's shield. Good hp start off and hp regain and armor for survivability. Ninja tabi, for dodge since my masteries are defense. I get this also, because I'll be getting a Phantom Dancer. This'll give me maximum dodge.
Phage - As you know, Mordekaiser doesn't have any slowing abilities. This will help non-squish him and give him some dmg.
Lifesteal - You all may wonder why I get vampiric scepter. As you know, Mordekaiser is a good farmer. If he builds this into a bloodthirster, he'll have a lot of damage. Lifesteal can also help replace his hp regain.
Phantom Dancer - Great atk speed, DODGE, and crit chance as well as movement speed.
Your last item could be one of these ; Atma's Impaler (Armor + Crit chance), Sunfire Cape, or Guardian Angel if you have a hard time surviving.
You may wonder, why I don't have ghost. Once again, Mordekaiser does not have a slow. His masteries can make his exhaust reduce armor and magic resist. Which is good for 1 vs 1ing.
I often like to hide in bush when getting chased at low hp and if the other enemy is low in health, I'll exhaust ignite and boom! They die.
I get my E maxed out first for maximum harass, armor, and damage.
The reason I don't get Q is that it doesn't do much at low levels.
Your W skill can replace a sunfire cape, because of the aura dmg and the armor and magic resist.