Since Mordekaiser got nerfed, i had to change the build a bit. His ability to gain shield from the dmg. he does to minions, was nerfed a lot, and is now harder to stack up. Therefore you need some Ability Power. Wether it's a Deathcap or a Rylias Scepter, isn't that important, as long as you get 150+ AP.

The Build:
Regrowth Pendant: In the early game, Mordekaisers abilities does take some HP. Mordekaiser is in fact, easy to slain the first 1-2 levels. But as he gains level 3, he's able to stack up his shield, and therefore stay alive. The Pendant makes it easier to stay lined for a long time, and since he doesn't use mana, HP is important.

Sorcerer's Shoes:
If you play well, and know how to last hit, you'll probably have 1100g in 4-5 minutes. Buying these shoes, will make you penetrate. Your runes makes your abilities hit harder, and the boots too. They are really important, as they will help, stack up your shield.

Hextech Revolver:
Cost 1200g. Gives 15% spellvamp, which means that you'll get 15% of the dmg dealt, back in HP. That's also good for laning with morde, or even soloing. If you want to solo, you could get the Hextech before the boots. And ofc. the 40 Ability Power, helps hitting down the enemy.
When reaching this, you should be around level 6. The combination of Ability Power, Ignite and ulti, will make you burst so much dmg, that you in a 1v2 fight, would be able to kill one of the two, and use the clone to slay the other one.

Warmogs:
Gives 920hp and some HP regen. Make sure to kill minions, as it will stack up to around 1500hp. That makes you a lot tanky, and with the amazing HP regen, you can stay alive for a long time.

Needlessly Large Rod:
You will now start building Ability Power for real. As it gives 80 AP, you will now have around 150-160 AP. (This is used later, for the Deathcap).

Thornmail:
Gives 100 armour, and 30% returned dmg to AD-players. Simply amazing.

Force of Nature: 76 Magic Resist (the item that gives the most) and 40 hp/5sec. Together with warmogs, you will now have more than 100 Hp/5sec. It also gives 8% movement speed.

Deathcap:
Deathcap will grant you 140 Ability Power, and also Increases your total AP by 30%. You will now deal some hard dmg!

Hextech Gunblade:
As we already have 15% spellvamp, we're now ready to upgrade that to 25%. Along with 75AP, 60DMG and 20% Lifesteal, your Mordekaiser build is now full!