I play Morde as a Beefy mage and I find myself owning almost ever champ. He gets very hard to kill with his shield and health mid to late game and even is quite annoying to kill early game as well. his shield can be problematic to people considering with this amount of AP it regens rather fast.

Pros

Alot of health late game so hard to kill
Amazing minion killer
Shield that regens pretty good with this AP
Can turn it to a 6v4 with his ult
You can tower dive pretty good (but be smart about it)

Cons

Rushing Warmogs means not alot of damage early so dont get cocky
CC pretty much screws him over but thats like all champs.
He can get annoying when he speaks =D
Late game you will be hard to kill but it will be harder to get kills as well but your carry should be able to help with that

I get magic pen in reds because when people start to get Mr its good plus its always good to have

I get Dodge in yellow because the less you get hit the better and you dont really need regen and If you dont have the IP for the dodge its also good to get the AP runes.

I find CD in blue is essential because that and blue buff and you wont need much more cause ur CD is pretty good at that point

In Quints i get AP cause its great to have early game But health is pretty good too.

My item build is straight health and AP

I get regrowth pendant first because it allows you to spam your spells and not worry about teh health you lose. With a dorans shield you dont get that OR health pots.

I get Magic pen boots for taht extra damage along with Void Staff And Abyssal Scepter

I think its best to rush Warmogs because Morde is a great minion killer so you typically farm faster then most champs so when you are able to get Warmogs they cant chew through your health like when you typically rush it on other champs.

After Warmogs its essential to rush Rylai's because its amazing for chasing plus that extra health. When you have your ult on them it slows the whole time and you can chase

I then get Rabadon's Deathcap to max out that AP for great damage. You typically can chew through squishy carries and take the damage at the same time.

I always get Ghost+Ignite.

Ghost is great to chase and Ignite is amazing with your ult and if done right you always get the kill.

Mid
He is a very good mid lane character because he is amazing at pushing. He is one of the best minion killers out there. So use your E to hit minions and if there is a ranged carry then stay back a bit and use your W on creeps ahead of you so it can do damage to people all around. also your W can be a good harass because it can hurt melee champs if you put it on a minion he is trying to kill.

1v2
He is possibly one of the best 1v2 to people beside Vlad. He can push those minions even against 2 people. I find myself pushing harder then them and getting more minion kills then mid. Dont be too stupid and try for kills. You are just trying to keep your tower safe and farm minions. With Morde i find it more important to get 200+ minion kills then dying trying to get that kill.

2v2
He is hell to go against alone and even worse with someone. I find it best laning with ranged on your team you will almost always win that lane and if you dont then you shouldnt be playing Morde cause he isnt for you. I have done best with teemos. they stay safe because the other team wont try and kill him. so you wont worry about a feeder teemo =D

By level 6 and up you should start getting jungle when you can and unless someone needs it more get Blue buff. Remember you dont have mana so dont be too greedy.

Like i said he is great at minion killing. You can t=kill whole waves with your E by level 9 assuming you had a good early game. Just hit them with your E and Q combo and the occasional W around you or your minion. keep doing this and you will be great.

when hitting champs its good to initiate and does pretty good damage.. follow it by throwing your W on you or a partner and remember it gives MR and Armor. then hit with your Q. by now you should have em low. Pop your ult+ignite and they should die... hit em with another E to make sure

Also remember while fighting. Your Ulti steals a portion of the damage it does so if getting away and hit with ignite it can save your life.

So to conclude. Remember Morde is a great minion killer and thats waht you should focus on to get your gold. If you feel like they are doing too much damage to you then a Abbysal Scepter is good as well. If its Magic or Zhonya's Hourglass for that armor. Please comment on how you think this build did for you cause i find myself doing GREAT with it so i hope you do too =D