Hii, this is my first guide, so, be nice, and give me comments on what I can do better, if there is anything I can do better that is. :)
I decided to make this, because, I feel that this build/guide is going to help many of you Morde-Lovers out there, and yes, I know there are many guides for him already, but I still think this one is going to help. ;)

Okay, I don't know why, but you have 425 Movement Speed, instead of only 385, because of Force of Nature

Maybe it would be much easier if I just wrote down all the stats instead? Hmmm...

I take the 9x Greater Seal of Resilence, and the 9x Greater Glyph of Magic Resist, because that extra Armor and Magic Resist is actually doing alot, especially when you get Creeping Death, becaues the skill also gives you Armor and Magic Resist.

Then I have Greater Quintessence of Ability Power for more Health.
I'm thinking maybe to get Greater Quintessence of Ability Power for more Ability Power.

In Offense Masteries, I pick Summoner's Wrath , Mental Force , Sorcery and Arcane Knowledge for good

In Defense, I take Resistance and Hardiness , Durability , Indomitable and Veteran's Scars , Initiator , Honor Guard and 1 point in Mercenary , just to get the 20 points needed to get Juggernaut .

Start with Boots and 3x Health Potion. This way you can use your abilities more often. If you don't get too much damage of course.

You should be able to last long enough to get money for Hextech Revolver, and Sorcerer's Shoes if you're good enough. But if you get money for the boots too, then head back to buy them.

Then go for Will of the Ancients, to get some extra Ability Power, and more Spell Vamp.

Rylai's Crystal Scepter is a must-have item as Mordekaiser, becaise, it gives health (needed), Ability Power (needed), and then it gives the bonus, where your abilities slow the enemy's Movement Speed (very helpful)!

Now you wanna focus on some more Ability Power, so Rabadon's Deathcap is the item you wanna go for. It gives 140 Ability Power, then it has it's UNIQUE Passive: "Increased Ability Power by 30%"! That really boosts your abilities ALOT!

Force of Nature helps alot, especially cause the enemy teams surelly have Ability Power-using Champions. It also gives good Health Regeneation, and the extra Movement Speed needed.

The last items you should get is Lich Bane for more Ability Power and some Magic Resist, and, it's bonus: "After using an ability, your next basic attack gains bonus physical damage equal to your Ability Power".
This bonus is very powerful if you use it with Maces of Spades to get most damage output.

I try to max the Siphon of Destruction first, because it has a good AoE (Area of Effect), which is very useful when farming minions on lanes etc.

But at the same time, you should try to keep Mace of Spades high too, because it really deals some damage, especially when you fight in 1on1 fights. And again, it's good to get minions! ;)

Creeping Death is not the best ability, and therefor not needed as much as the others. But still, it's very useful.

And for Mordekaiser's ultimate, Children of the Grave, this skill you should try to put on their Ranged Attack Damage Champions. These are the best to have as a "minion". This is for example Ashe, Graves, Twitch etc.
If you can't get one of those, then get a Melee Attack Damage Champion, because they can deal some damage too. This is for example Master Yi, Tryndamere, Sion etc.
Don't use it on Tanks and Mages, because their Normal Attacks are pretty useless.
Only use it on Tanks, if he/she is the last alive, and you can use the "minion" to tank towers with, or Baron etc.

Pros:
-Lasts long (moderate) in teamfights if not focused too much.
-Very good at farming minions.
-Almost every ability is AoE (Area of Effect, which means they deal damage to multiple targets).
-Able to turn teamfights very quickly with his Ultimate.

Cons:
-Consumes health to use abilities.
-No CC (Crowd Control (Slows, Stuns, Binds, etc.))
-Enemies can quickly escape you, if you don't have Rylai's Crystal Scepter.
-If stunned before being able to cast any abilities, he's an easy target.

Take Flash, bacuse it is the best Summoner Spell for an Ability Power Champion, who doesn't use Mana. You can easily surprise enemies, and you can easily get away from unwanted fights and chases.

Ignite is a good comination with Children of the Grave, because when the enemy tries to run, you just put your Ultimate and Ignite on them (when they are low on Health Points of course), and you should be able to get the kill, and the "minion", which you now can use to backstab the rest of your enemies with.

When in teamfigths, try to focus on their Ranged Attack Damagers, or Melee Attack Damagers, because they are very dangerous for you and the rest of your team. And they goes down pretty fast if you focus all your abilities on them first. Plus, your team can just massacre all other enemies at the same time.

And for God's Sake, don't ever steal your team's AD Carry. EVER!
They need the kills more than you, so let them take as many kills as possible! And try to defend them as much as you can.