The purpose of this build is to make you really off-tanky. In other words, you'll be dishing out lots of damage and still never die. The fun thing about spell vamp on Mordekaiser is that it goes very well with his shield. His shield already blocks damage based on how much damage his abilities do. So, with spell vamp, his abilities are both blocking and regenerating his health. This means that, as long as he keeps using his abilities, he will be very tanky while doing plenty of damage in the process.

-updated build for new patch-

MR/level Quints/Glyphs - More magic resistance, makes your shield last longer against magic damage and makes it easier to spell vamp your health back up, since you won't lose as much.

Armor Marks/Seals - More armor, for the same reason as above, except physical damage instead of magic damage.

9/21/0

The defensive masteries make an already-hard-to-kill champion so much more annoying. The nine masteries on the offensive tree are all good on him as well. Alternatively, you could go 21-9-0. The defensive masteries are generally better on him though, especially Tenacity, which makes it harder for opponents to penetrate your shield.

Rejuvenation Bead+Health Pots - This gives you the ultimate sustain in lane. Be careful though, Morde is still squishy early game, so you can't be too aggressive.

Other Options-
Boots of Speed + 3 Health Pots - If I were ever to start Boots on Morde, it would be for the health pots. He really doesn't benefit much from speed early game.

Regrowth Pendant + Health Pot - This is a solid start on Mordekaiser. I prefer Doran's Shield though.

Amplifying Tome + Health Pot - This will give your shield a boost. It's not really effective enough to be a good first item though imo. I'd rather take the health regen.

If you're going for AP and Spell Vamp, you should probably get Sorcerer's Shoes. The magic penetration helps you kill a lot faster. Other good options include Mercury's Treads, Ninja Tabi, and Boots of Swiftness.

- A quick Revolver eliminates the health cost from your abilities, since it heals for about the same amount they cost. It also allows you to fight a lot better and longer.

- This gives you a little extra health and ability power, along with magic penetration. You'll sell this later, but it's a great early game item for its cost.

- Rylai's gives ability power and health, both are great on Morde. The slow is simply awesome with Morde's E and allows him to chase very well. It's also a must-have since Mordekaiser has no built-in cc.

- The movement speed allows you to chase and get away from fights very well. The magic resistance and health regeneration gives you much more sustainability and makes you much beefier against mages.

- Frozen Heart may seem weird on Morde, since it gives lots of mana, which Morde doesn't use. But, it's other stats, the armor and cooldown reduction, are great on Morde.

- Everything about Hextech Gunblade is just awesome. Spell vamp and life steal are very good. They are now a necessity for a nontank Morde, due to his recent nerf. Then, you also get attack damage, life steal, and an awesome active that goes great with your ultimate.

- Deathfire Grasp is a bit inconventional, but very good on Morde. Cooldown reduction eliminates the couple one to two seconds of you not being able to recharge your shield. It also has an amazing active that goes great with your ultimate. Using this item's active along with the Gunblade's and your own abilities and ultimate, you will have a ridiculously high amount of burst damage and can turn a team fight instantly into a 6v5. Also, it gives a bit of mana regen for good luck.

Here are some other viable items for this build.

+ - These two items could replace the Frozen Heart and Force of Nature. Guardian Angel has both armor and magic resistance, making it very good. Along with it, I would recommend an Abyssal Scepter for more magic resistance, ability power, and penetration. You could replace the Guardian Angel with an Aegis of the Legion and the Abyssal can be replaced with a Banshee's Veil.

- You can get a Thornmail instead of a Frozen Heart. This is good if you're getting targeted by AD champs.

+ - If you're getting fed or are somehow able to carry hard, you can replace Frozen Heart and Force of Nature with these. You may need a very reliable tank to pull this off though, to keep the other team from focusing you.

- Just getting one or even two more Hextech Gunblades is also a viable option. If you're fed and want to be able to solo teams, this is how to do it.

+

Almost all Mordes go Ghost and Ignite. This is for good reason. Ignite is a must have, because it goes extremely well with your ultimate. Ghost is also a must have, since you lack cc and have to chase to get many of your kills. Flash could work too, so you can get in the middle of team fights in an instant or get in better position to ult.

Morde should never play as the main tank. He lacks cc and therefore sucks at initiating fights. Let someone else on your team play as the main tank, initiator, w/e. Then, you go in second and start destroying everyone. You also shouldn't be the carry, since Morde's shield works extremely well for tanking. Morde is definitely an off-tank and should always, with no exceptions, be played as one.

new patch - Morde's best new role is as a troll. :D

Chaser - Ghost and Rylai's make you a great chaser.

Meatshield - Thanks to your passive (the shield), Mordekaiser can potentially be the best meat shield in the whole game. Not anymore!

Making Ghosts - Build some ability power, target the carries, kill steal if/when needed. Just get as many ghosts as you can. A good ghost can make a huge difference in the game, especially if it's a fed Tryn or something. Remember to use ignite and the gunblade's passive along with your ultimate!

Remember to not charge in without getting your shield up!