Heres how i personally play mordekaisers build.
I just wanted to share this with you guys. This build works for me.

Mpen gives you some edge in the beginning. You're mordekaiser and you must keep enemies away from creeps with siphon of destruction. Armor runes. well no reason. You just can endure more ad dmg in the beginning. Cooldown runes? If you ask why you should have cooldown runes you should stop playing mordekaiser.

Masteries i've added here are my personal masteries. Copy them down or figure out your own favorite masteries that work for you.

Ignite and exhaust? yes. Ignite with your ultimate equals a terrifying combination. Exhaust for slowing enemies down and giving you an easy kills.

Start of with Regrowth pendant and health potion.
Next continue warmogs by buying giants belt and if you can finish it as early as possible.
If you cant finish warmogs early you should start building boots with boots of speed.
After warmog is ready buy sorcerer's shoes and start rylais crystal scepter with amplifying tome. After its ready the rest of your items you buy according to situation or how offensive you wanna be. I usually play quite offensive but i like to endure lots of punishment. Morde's shield is great but magic resistance never hurts. Guardian angel is great. You need to be killed twice every 5minutes and you have mres and armor. Sunfire cape for hp and magic dmg for surrounding enemies. warmog for hp and regen. Rylais? HP and AP with a slow. niiiice.