First off I'll let you know that this is my first guide. I main Morde, and he is one of the funnest champs I have played. This is my guide, hope you enjoy.

Mordekaiser is a unique mage, very tough compared to most mages due to his passive. His ult can also turn the tide of any battle if executed right.
I am going to assume you can figure out how his abilities work, so I am going to skip the skills explanation.
Also, I view spellvamp as a VERY important asset to Mord due to his health cost at early levels, and because all his abilities are aoe, meaning in those late game teamfights you can jump in there and be a god among men due to massive spellvamp coupled with you shield and healing/ghost from your ult.

I choose quints of fortitude because of of the extra tankiness you get early game. Also, Mord uses his own health as a resource for spells, so early game can be tough for him. These help offset his own leaching.

As for Marks of insight, I view these as essential on almost all mage type characters, and Mord is no exception. They let you do that little extra damage that is crucial

As for my seals of defense and glyphs of shielding, well Mord is unique as a mage being a melee fighter and have shoot range spells. This means that most of the time, in order to be effective you are close to the enemy. These help you stay offset some of that extra damage you should be taking.

My masteries are rather simple. Standard offense for any mage, with a little extra tankiness from the defense tree, this being because Mord doesn't really benefit from anything in the utility tree. The extra health and armor/magic resist really help Mord out too. The extra health is great in the beginning when you really feel his self abusive abilities the most. And that extra armor and magic resist are essential considering you are a mage, but also a melee fighter so you are in the middle of things and need to be able to take some punishment.

One thing that makes Mord very unique is the fact that all of his abilities are AOE, also his passive which gives him a shield. The fact that all his abilities are AOR mean that spell vamp is AMAZINGLY effective on him. Also, spell vamp is essential to counter balance the health cost of Mord's abilities.

The Fact that spell vamp is so effective on Mord is why you ALWAYS rush Will of the Ancients.