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 build has worked wonders for me, I have never lost a ranked match as Mordekaiser using this build. I hope you give it a try and I hope it wins you many games.
The runes in this build give you some early game and late game tankiness that will allow you to stay in your lane and make it so that you can survive some harassment from the enemy. The Ability power and magic pen, give you the ability to get your shield up quick and harrass the enemy/farm to your hearts delight.
Although you might be tempted to grab a point or two in your creeping death, this will slow your damage output from your 2 main skills, mace of spades and Siphon. If you feel like you are getting too much harrass and can't stay in your lane then just stay back a little and don't get too close, try to prevent yourself from spamming your skills so you can regen your health, the creeping death is not worth losing some much needed damage and DPS.
Creeping / Jungling
You can easily start to grab a couple minion camps in your free time if your enemy is sticking behind their turret like glue. and they probably will because you are going to be almost instantaneously killing their minion wave before it starts to attack. Although you might think that pushing is a terrible Idea, as long as you have a good team that can shout MIA well, and if they can push to the enemy turret as well, you will be having a great safe time harassing them at their turret.
This build ensures that you can stay in your lane indefinitely untill you need your items, sometimes I find myself in lane so long that I have enough for Force of nature on my first trip back. If by some chance you are too low health to stick around for more cash, you will want to go for a second Regrowth and boots.
I don't have the pictures but I have gotten a full match history full of Morde wins with this build. I have also never lost Ranked as Morde, check my ranked if you don't believe me. My Summoner name is Therodes. Add me and I will be glad to give you a 1 on 1 tutorial on how to play morde more efficiently.