This build focuses on maxing out the CDR cap and cranking AP in order to gorge Iron Man's shield. If the best defense is a powerful offense, then the greatest would be Morde eating yer face...

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My Morde build has a variety of delicately balanced goals, in order of priority:

*1: Reach the 40% Cooldown maximum.
*2: Accrue as much Magic Penetration and Ability Power as possible.
*3: Accrue as much Movement as reasonable.
*4: Maintain a high degree of Damage Mitigation.

The first two exemplify the Morde philosophy of the greatest defense is a monstrous offense. The Mastery, Rune, and Gear selections I have assembled provide a nice synergy towards these goals.

The CDR breaks down as follows:

Masteries:
*Sorcery: 3%
*Intelligence: 6%

Runes:
*9 Celerity Glyphs: 8.1% @ lvl.18
*1 Celerity Quint: 2.34% @ lvl.18
*2 Focus Seals: 0.58%

This comes to 10.16% @ lvl.1, and 20.02% @ lvl.18, plus...

*Morello's Evil Tome: +20%

So once this is bought, you'll have somewhere between 30.16% and 40.02% CD.
The blue buff or potion can compensate for the 10% gap before lvl.18.

It's insane trying to spam the abilities fast enough when the full 40 is in effect...

Runes give 10 points, Sorcerer's Shoes give 20, and the Abyssal Scepter effectively gives 20 more. I prefer it over Void Staff. They have the same AP, and the Staff has 20 more MP, but the Scepter's MR is crucial, and the effective MP from it is an aura that works for my allies too. I'd love more, but fitting in Void Staff or Haunting Guise harms the balance of the other gear traits, or removes Rlyai's Crystal Scepter, which is great crowd control for when you're all decked out for destruction, and CC is sorely missing from Morde's skill set.

With the patch a while ago, Morde's use of Ability Power has been fixed. For this reason I also have a more even hand buying the abilities up than other guides. This build has sought a high degree of AP in combination with the other goals, and with the exception of the Sorcerers Shoes, all of the final items have a significant AP bonus, allowing it to clear 400!

Though I only have one movement Quint due to liking rounded values in MP and CD, and I prefer the Sorcerer's Shoes over Boots of Swiftness, I still make up for a good amount of speed with Lich Bane, which provides a 7% increase. Rylai's Crystal Scepter also helps to mediate any speed disadvantage. In an emergency... FLASH!

Though Zhonya's Hourglass, Abyssal Scepter, and Lich Bane are all fairly expensive, the component items that give them most of their DM are quite affordable. I buy either a Chain Vest or a Negatron Cloak on my first trip home, depending on who I'm laning against. Then it's either a Null-Magic Mantle or Sorcerer's Shoes, both cheap enough to buy early. Even buying those pieces to their full versions will barely provide better ratings, though the added AP, etc. is great.

All in all, this is a bit less Armor and Magic Resistance than most builds, but in my experience, the bloated AP, MP, and CDR account for tons of incoming damage. With Children of the Dead, you can not only regenerate any damage that washes over Iron Man, but you can reverse the odds of a two vs. one fight in your favor.

Without the common Doran piece, and a utility-based mastery build, I play very conservatively in the first few levels. I rack up on health pots for the early farming until I can afford the first of my defensive pieces, either Chain Vest or Negatron Cloak. After both of those, Sorcerer's Shoes, and a Null-Magic Mantle, I start to play more aggressively.
The Awareness mastery will push me ahead just enough to be helpful as long as I protect myself while still squishy. With cranked CDR, AP, and MP, I start to kill multiple opponents surprisingly fast. With the CDR and the passive shield ability, coupled with Children of the Dead, I can stay up with a thread of health and pentakill (or have before, anyway...)
I like to round out this build, after a few stat pots, with Rylai's Crystal Scepter, but a Void Staff would be brutal if you were confident in keeping up with the enemy.

If you like this build, I know how to drive it, so friend me in game:

DCLXVI the Beast

(wouldn't allow me to take that handle on this site...)