Okay, I posted this a while a go and it didn't do very well. For the following reasons. 1. Most people think of morde as a high survival tank. While this is somewhat viable, its just not how i play him. 2. One of my buddies regularly plays Rammus. (Check out Primevilraam's Rammus build!) since he is never more than a few seconds away, i don't really have to worry about survivability. Even if you don't have a selfless Rammmus buddy, its okay because of Mordes passive. The more damage output you have, the faster your shield charges, the more survivable you are. Also your ult gives you quite a bit of health and shield over time, given enough ap. Therefore, even without tanky items, you still are more difficult to kill than any other burst caster. Finally, YOU SHOULDN'T GET THE CHANCE TO FINISH THIS BUILD!!!!!! Morde should be able to 3v1 most of the time after Zhonia's. You pick the squishiest enemy, ult, ignite drop the chain, pick up the ghost, and proceed to terrorize the other two opponents. One last note, don't knock it before you try it. I swear its usually super effective.

Most people think of morde as a tank. This, is stupid. Morde has so much more potential as a nuke, getting a ghost during team fights, and pushing all the way through a lane. I've played morde for several months now and this is what works best for me.

Ok, magic pen. marks are a no brainer with morde. They make him hard to lane against early game when combined with Haunting Guise, because they preserve much of his skill damage, which also equates to a large shield sooner. I take flat AP quints for early game once again trying to maximize early damage. Next, I take life per level seals to give morde some late game survivability. Finally, cool down per level glyphs let me dish out a little more punishment late game.

Not much to say here, I built straight AP damage. what you do with the other points can match your tastes.

I like to open with haunting guise, which in my opinion was made for our buddy morde. It gives him a little health, a little AP, and that ever important magic pen. Next, we grab our sorcerer's shoes, which gives me movement speed and more magic pen. Its now a few minutes into the game, and we already have 50 magic pen. In other words, you can destroy squishier carries with your skills. In the event that some one gets smart to an AP morde with magic resist, abyssal scepter and void staff are great options. If someone with a lot of CC is giving you trouble, moonflair spellblade is an awesome option. Once my magic penetration is secured, I simply stack deathcaps, this is aimed at making mordes skill do tons of damage to charge up his shield. By endgame, you should fill your shield by firing off just one of your skills. AP also builds up Children of the grave's percentage, which makes morde very useful in quickly destroying tanks, even during the endgame. Finally, many people like to try to add spell vamp onto morde, however I find this ineffective because all mordes skills cost life, which basically negates the effectiveness of the life gain. You're a lot better off just building straight AP.
*Update-I now throw in a crystal scepter so I can usually survive/win 3v1 ganks.

Pros:
Skills do tons of damage to fill your shield quickly
Farms like a boss
Lots of magic penetration
Useful for killing high Hp enemies
Lots of AOE
Shield gives you decent durability
No mana to worry about

Cons:
Squishier than other morde builds
Too slow to really escape ganks
You become a priority for assassins
Potentially disastrous early game

Ok, I like ghost as my GTFO skill, most people like flash, but i prefer a constant sprint over some thing that can go wrong because you didn't have your mouse pointing to the right place. I also find it more useful for getting to team fights in time to be helpful. Finally, in the event that the laning phase isn't going so well, the extended sprint can help you to kite the other champs.

Ignite is the perfect complement to morde's ult, giving you great DOT, often just enough to get that all important ghost. Its also useful for getting annoying champs like Xin, or Jax off your back early game.

Early Game:
Open with Amplifying Tome, and build it into a Haunting Guise, and pick up sorcerer's shoes ASAP. These will allow you to make any enemy champ keep their distance and fill your shield very quickly. Always try to keep your shield near full, seeming as often that will decide whether or not you get ganked early game. This is easily accomplished by leveling up siphon of destruction as early as possible, and grabbing your ult to pick off low life enemies.

Mid game:
Here your goal is simple, farm FARM FARM! Repeatedly nom through the enemy creep waves picking up as much gold as possible, and picking up the occasional kill if possible. If you are having trouble with taking damage from carries, you can turret hug and throw creeping death onto minions to farm and harass from afar.

Endgame:
Now, after achieving your first deathcap, you are powerful enough that you should be able to 1v1 just about anyone. Your high magic pen. lets you preserves almost all of your damage on all but the tankiest champs. You should also be able to drop huge amounts of AOE damage on team fights, simultaneously filling your shield and taking down the enemy's health. After the second deathcap, you should be powerful enough to plow through enemy creep waves and push your lane extremely hard. If the other team attempts a 2v1 or a 3v1 gank, immediately pop your ult on the highest damage output, and fire off siphon, using ignite as needed. Then, use creeping death on yourself followed by mace of spades. by that point the squishy ad champ you tagged with your ult should have died and produced a ghost you can use to help fight the other two another siphon and the ghosts attack should be enough to kill or chase off the other champs. Resume pushing your lane after recalling if necessary. If your team is having trouble taking down an enemy tank, you can pick up deathfire late in the game and use it to replace Haunting Guise. Firing off the 30% nuke, right before using your ult, you can take down any champ in seconds.