Pros:
-One of the best farmers
-One of the best pushers
-Can win 1v3 fights
-Amazing ult

Cons:
-Squishy early game
-No escapability, which means that the enemy jungler can completely shut you down

I go with magic pen reds, AP yellows, AP blues, and health quints. These runes allow you to have great early game damage, and a bit more tankiness.

I go 21/9/0. This gives you great damage throughout the game, which translates into more tankiness (More damage = more shield and healing)

Always start with a regrowth and a pot. This will allow you to spam your E without losing health, which is necessary for your harassment and sustainability in lane. Next rush a WoTa, a hextech revolver, and sorcerers shoes. This will give you 40% spellvamp, and decent damage and magic pen.

After getting spellvamp you should be able to completely bully top lane and push down the turret. After doing that, roam a bit and try to get the other 2 outer turrets if they're still up.

Your next item is Rylais, which should be followed by a Rabadons. At this point you'll be dealing enough damage that you can basically just run into the enemy team and pick up a 1v3 triple without too much trouble.

After a Rylais and Rabadons, I like to build a Zhonyas Hourglass. This is a great item and I build it on almost any AP champion. It gives you great AP and a lot of armor, and it's got a great active to use while you're waiting for your abilities to go off CD

After Zhonyas, you should finish your build by building your Hextech revolver into a Hextech gunblade. This will give you an insane amount of spell vamp and it will cause your Q to do tons of damage.

I almost never have to change this build, and if you play Morde right you shouldn't have to either.

Always get ignite on Morde. when you combine it with his ult it should burst down your opponent for almost half their hitpoints. For my 2nd summoner spell I usually get exhaust. It gives Morde CC and allows him to bait whoever you're laning against into thinking they can kill you. Flash and ghost are also viable, because Morde is extremely slow and has no escape/chasing ability

Morde is IMO the best farmer in the game. By using your E to instantly kill the ranged minions, and then using your W and Q to kill the melee ones, you should usually have about 120 CS by 15 and 250-300 by 30. The only downside to farming like this is that it makes you very open to ganks, so I like to hide in the bush after I push the minions to the turret until the enemy minions come back. Also, make sure you ward so that you can run away before the enemy jungler ganks you

I don't see a reason to build Morde as a tank. He has no CC, and he isn't as tanky as some other champions. Also, if he gets fed enough, he can be more tanky as an AP bruiser than as a tank, if he doesn't get stunned.

Although Morde was nerfed to the point that he was completely useless, because of recent buffs, he can farm well enough to still be able to have decent damage and tankiness. If played correctly he can still be a top tier champ.

This is my first build so please give me some constructive feedback. ("u suck" is not constructive) :P