Here im going to talk about the Runes & Masteries and why i choosed just these Runes & Masteries

Runes:

I picked these runes since it contains everything you need in Laning Phase and Late Game.
The Pure Ability Power Quints is there because you usually use that on Magic Damage Champions and It works good with Mordekaiser. You can actually change the Quints with Spellvamp Quints to sustain earlier in Laning Phase, otherwise you don't really have any sustain early on besides Health Pots.
The Defensive runes is there to actually survive in Laning Phase against Champions in Laning Phase.
The Penetration Marks are there just to deal some extra damage to the Enemy.

Masteries:

Not really much to talk about here. These Masteries are used by most of the Magic Damage Dealers in Mid Lane Especielly. The Masteries brings Magic Damage and some Tank Stats.

You might Wonder why I bought some of these items or why i picked Ghost instead of Flash? Well here are the answers.

Summoner Spells:

Ignite is there for Kills in Laning Phase, it makes it easily to get kills in Lane.
The Ghost though might seem like a wierd pick but it actaully works perfect with Mordekaiser. I have expierienced it in League of Legends and it works better then Flash if you're playing really Defensive in early Laning Phase. The Ghost is there for the Mid to Late Game. To have the ability to just run into the enemy team With your Passive's shield and all your Damage is a huge Advantage for you.

Items:

At first when the game starts I usually starts with Dorans Shield + Pots against ranged Champions, if they are meele though i start with Pink Ward + Bead + 5 HP Pots which gives you great early vision and Sustain. After this I always start with the Hextech Revolver to get that sustain early in Laning Phase which helps alot. After that I pick off something that you don't usually see Players pick of in Mid Lane. The Spirit of the Spectral Wraith, i buy this since when i use this build on Mordekaiser i farm the Minions in Mid Lane and the Wraiths constantly. If you are not farming the Wraiths as fast as they are spawning it's not worth to start with the Spirit of the Spectral Wraith. After i have gotten the Spirit of the Spectral Wraith i Finish the Hextech Gunblade for Some more Damage and the slow. Void Staff fits Great in Mordekaisers build aswell since it gives you that Extra Damage and the Magic Penetration is also great. Deathfire Grasp is the item I usually pick up after that or I go with the Deathfire Grasp and then Void Staff. Deathfire Grasp gives you the Ability to really burst someone down which can be really game changing. If im ahead i usually build the early to mid game as i have explained above but if im behind i build in another way. Then i start the same with Hextech Revolver and the Spirit of the Spectral Wraith but then all changes. Zhonyas Hourglass is the item i follow up with and after that I buy Abbysal Scepter for The Resistence. Rylais is also great with Mordekaiser for that Extra Health and don't forget its Passive. The Passive slows target's hit by Abilities which means that you can easily chase enemies or slow them while getting chased. After all this I Usually pick up Rabadon's Deathcap on either Builds. I do that because all damage you get from it is sick!

Of course all Champions have Pros & Cons so im gonna count up the Pros & Cons with Mordekaiser.

Good Things:

High Burst [If he puts all his Abilities on single targets]
Easy Farming Techniques
Passive Shield Good for trades in laning phase
Mixed Damage [HP % Based and Magic Damage]

Bad Things:

No Escape Ability
Easy to Counter [For Example QSS For the Marksman]
Lack of CC [Your team needs to make up for that]
His Abalities can really only pick off 1 Enemy

How do you farm with Mordekaiser efficient, and how do you farm without losing to much HP.

So in Lane you want to be careful in early level since your Damage of your Abilities are not producing so much into your Passive's Shield. When you have gotten some levels though you can start put yourself in the middle of the enemy Minion Wave. So you wanna put the Creeping Death on yourselfe, then use Siphon of Destruction on all Minions in the Wave. Following that you wanna use the Mace of Spades on the Closest Minion. If you wanna know how you can Increase your farm and money you can go to the "Farming with The Spirit of the Spectral Wraith" Chapter.

You might Wonder how often i clear the Wraiths and when I do it, well here is the answers.

So as everyone know it spawns a minions wave every 30 Seconds. There is 1 Minute Respawn time on the Wraith Camp. This means that you can push 2 Minions Waves really quick in mid lane then go to Wraiths, Clear it and return to mid. When you have returned to mid you clear 2 Minion Waves and then Wraiths again. This will result in around 15 Last Hits a Minute which is around 520 gold on 1 1/2 Minute which is a HUGE Amout of gold. This is with the Spirit of the Spectral Wraith though. It's 400 without the Spirit of the Spectral Wraith which is also great.
This is a great way of earning fast money and get all your items faster then your Enemy and become a HUGE Threat!

Mordekaiser is not the easiest Champion in the game and some people have no Idea how to lane Against and With him.

Laning With Him:

You have to pay attention to if your Passive's Shield is up and how much damage it can absorb
No Escape abilities besides Flash [Which means the enemy Jungler is a big Threat]
If you get Ganked you might be able to 1v2 them since your Passive's Shiels is Powerful
Vision is Important for You since you have no Escape Abilities

Laning Against Him:

Don't Underestimate the Power of Mordekaisers Passive's Shield
Always pay Attention to His Passive's Shield
Mordekaiser is a Huge Threat after Level 6
Deny His Vision in Bushes in Laning Phase

How to position Yourselfe in fights and who should you Deal Damage to?

Since Mordekaiser got no Mobility Abilities i run Ghost on him to actually get into the whole enemy team when fighting. You can either team fight in the game or pick off enemies in hidden spots or in the jungle. Mordekaiser is difficult to teamfight with so you gotta be careful but at the same time you have to run into the enemy team if you wanna team fight. When the team fight is about to break out put Creeping Death on yourselfe and use Ghost to close the range between you and your Enemy. When you are in their whole team you have to sustain and actually survive. You should try to Explode their Carry. If you put your Children of the Grave on their Marksman and get her as your pet you have a huge advantage in the fight since your pet will deal much Damage. Since Mordekaisers Ultimate is based on % HP you can also try to put it on the enemy tank such as tank laners, this will result in Massive Damage. When you got 3 points in Children of the Grave your Ultimate does around 45 % of their total HP.

If im gonna wrap this up i wanna say that you have to be really careful and dont overextand to much in Teamfights.