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.
Well this is how I Mordekaiser. Ive gathered a good amount of information from all around and finally came up with this. I don't know how to classify this, though I just say its an AP/DPS/Tank hybrid I guess.
Start with a Regrowth Pendent and a Health Potion.
The Regrowth Pendent covers Mordekaiser's health costs quite nicely and the Health Potion is helpful for if you get attacked with your shield down/ganked, and all you need to do is stand back and heal up with it, and the health regen you have will help as it is used.
Once you have enough gold and if you're lane is not in danger, get the giants belt for 1110 gold, or if you can hold out longer, get that and a pair of boots as well.
I would not suggest getting the AP items until you can finish the Rylai's Crystal Scepter unless you am forced to back before you can finish it, and even then you should finish the boots before it even if you could because the early extra Magic Pen. works wonders for your shield, especially early game.
Finish both your Sorcerer's Boots and Rylai's Crystal Scepter, then start on the Hextech Gunblade.
If you are able to finish your Hextech Revolver in one go with 1200 gold then get it, if not, get the Vampiric Scepter first, and they the Hextech Revolver right after.
After your Hextech Gunblade is finished, work on the Sunfire Cape.
I suggest starting with the Giants Belt for the extra health over the Chain Vest, as your shield will eat up any damage you will take, and the small less decay from the Chain Vest is not really worth it over the health behind the shield.
Finish the Sunfire cape as soon as possible, and, if the game isn't already over, start your Malady.
For building the Malady, you can get the Amplifying Tomes over the Dagger since that small amount of attack speed does not out weigh the extra damage from your spells.
Start and finish Warmog's Armor, win game.
Even though you probably won't finish farming up the 100 minions to get its full bonuses, though the Champion kills and assists will fill it faster, it is still a huge bonus to have at the end of the game.
I use to start with a Warmog's but decided I wanted something to do with that health or something else on the item with it. Of course getting the sorcerer boots to help with magic penetration to get more damage from your abilities, and combined with the runes it hits even harder. The Gunblade is amazing as it keeps you going in a fight, and the spell vamp is a great part of it. The Sunfire Cape helps for its armor and health, and the passive goes great with Creeping Death, though it is already 2 of them at level 5, but if you're facing a more heavily magic damage team you could get a Banshee's veil for the extra health, Abyssal scepter for more AP and reducing their magic resist even more, though no more health, or even a Hexdrinker I suppose if they're more of the nuking kind. I added the Malady for more attack speed and to reduce their magic resist even more, though I have often considered getting a Nashor's Tooth for the cooldown reduction, but this just seems better since his cooldowns are pretty low already. The Warmog's is just for the great boost of health at the end of the build.
The summoner spells are typical, and mandatory, for Mordekaiser. Ignite to go with your Ultimate, and ghost to catch up to someone, or to run away with his lack of escape abilities.
The masteries I felt were better than going down the defense tree. Getting the summoner spells down faster, move speed, exp, and a bit more cooldown were better for not being a complete tank.
The runes are also very helpful to Mordekaiser. The magic pen. runes to go with the sorcerer boots are the main part of them. The cooldown glyphs I don't really know what else to use, and I don't have any other myself, but they go with the masteries quite well. The magic resist seals I don't own myself but they are what I would get if I made a page just for him since the main build does not include much Maric Resist. The Quints could easily be switched out for some move speed runes, which I am considering doing my self since I stopped getting Boots of Swiftness on him, and I do miss the faster movement.
I have used this build a lot and it works wonders. Though it lacks a straight focus, I believe that this 3-way hybrid thing is an amazing way to build this amazing and unique champion.
And to show how this build can kill everything, and completely brag about it, here are a few pictures I have of it (yes I go by Jedrek13 in game):