Welcome to my Mordekaiser build, focusing on middle lane play. I've been playing Mordekaiser for ages, and this is the best way to build him in my opinion. Please read the entire build before voting it, and do vote please :).

Here are a few results I've had with this build.

PASSIVE: Iron Man. 25 / 27.5 / 30% of the damage dealt from abilities is converted into a temporary shield, absorbing incoming damage. Maximum shield strength is 90 + (30 x level). The shield decays by 3% per second.

This is what makes Mordekaiser the beast he is. I always refer to this as the ultimate protection to harassment. If you can keep this shield up during teamfights you'll be granted with alot of survivability.
Q: Mace of Spades. On next hit, Mordekaiser swings his mace with such force that it echoes out, striking up to 3 additional nearby targets, dealing magic damage. If the target is alone, the ability deals 75% more damage.

I find myself rarily using this untill I fight someone. For farming this isn't nessecary but it's nice if creeping death is on cooldown or just if you need some extra shield life.
W: Creeping Death. Unleashes a protective cloud of metal shards to surround an ally or himself, increasing their armor and magic resistance and dealing magic damage per second to enemies in the cloud for 6 seconds.

A truly amazing ability for farming and to feed our ultimate solution against harassment. Constantly damaging nearby creeps will assure you of a lasthit if your actual attack is just a bit off.
E: Siphon of Destruction. Mordekaiser damages enemies in a cone in front of him, dealing magic damage. For each unit hit, Mordekaiser's shield absorbs additional health.

This ability is amazing. It boosts your shield up by a ton if you hit multiple creeps, this'll be the main ability to feed our ultimate solution against harassment. This will be the ability we max out first because of the shield and the amazing farm.
Ultimate: Children of the Grave. Mordekaiser deals magic damage to the target and curses it for 10 seconds. The damage is equal to a percentage of the target's maximum health. Mordekaiser heals himself for the damage done. In addition, if the target dies while under the effect of the curse, their soul is enslaved and will follow Mordekaiser for 30 seconds. During this time, Mordekaiser gains 20% of their ability power and attack damage. The spirit gains 75% of Mordekaiser's adjusted damage, ability power, and health totals.

In combination with ignite, the damage over time is AMAZING. This ability can be used both for defense and offense. I highly doubt this will happen with this build, but if you find yourself running low on health and you're being chased, cast the ability on one of your chasers and you will heal yourself for the damage it deals. Most of the time I use this to deal the last bit of damage together with ignite. For example when the enemy thinks he's safe with 2-3 bars on the tower I cast both ignite and my ultimate and we'll get the kill.

Mordekaisers Stats:

First we'll be maxing outSiphon of Destruction. Why? Amazing farm, and it boosts your shield by an amazing amount. After you casted Siphon of Destruction on the enemy minion wave you'll have enough shield life to protect yourself from incoming harassment. Next we'll max outCreeping Death. This will protect your carry in teamfights, farm you alot of gold, and give your shield lots of life. The constant damage will slowly regenerate your shield when you're nearby enemies. This will assure you some lasthits when you're actual attack is just a bit off, as I've mentioned before. Last but not leastMace of Spades. This will feed your shield in teamfights, deal lots of damage, and give you some farm. Obviously you level your ultimate whenever you can :).
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I go 9/0/21, taking improved ghost, health regeneration, increased movement speed, 1 gold per 10 seconds, and cooldown reduction. Cooldown reduction is nice on every champion, and it's nice to be able to useSiphon of Destruction twice per wave so you get some additional gold and faster pushing. Creeping death has a very long cooldown so this also helps. Health regeneration because your abilities use health and you'll be spamming quite alot when farming. It's also nice to have health regen so you can stay in lane longer. The movement speed is pretty self explanatory, catching up with enemies, running away... Especially with Force of Nature this'll help alot :). Improved ghost, also self explanatory. It improves your ghost, need I say more? Same story for greed. I put a point in this because it's better than the 2 mana masteries, since Mordekaiser has no mana.

Mordekaiser is the best mid character if you ask me. His shield prevents him from being harassed and his farm is amazing which also makes him a great pusher. As soon as the creeps show up, useSiphon Of Destruction, which will give you a nice boost on your shield, now useCreeping Death, which will constantly regenerate your shield when you are near enemies. With your shield your enemy will be unable to harass you and you'll have the minion wave cleared quickly. Watch as your minions attack the tower and if you feel safe enough attack the tower aswell. If you're not attacking stand behind your minions, and when the enemy comes close useSiphon of Destruction. If you're being ganked, use ghost and attempt to escape. In most situations your shield and the fact that you are a tank will keep you alive untill you reach the safety of your turret range.

If you find yourself in a 1v1 fight, you'll pretty much always win. Even though we're building nearly fulltank, the damage from rylais will help. Ignite andChildren of the Grave will constantly damage them, together with sunfire cape and creeping death. Between your abilities don't forget your basic attacks, they might not deal alot of damage, but it's something :). When your enemy starts running, the slow from creeping death (rylais crystal scepter) and your other abilities will make sure he'll not away.

During teamfights your main goal should be to protect your carry. Whenever it's up cast creeping death on your carry and block incoming skillshots. Once your team is winning and the enemy team is starting to back off, move to the frontline and now you REALLY start doing damage. With your Force of Nature and improved speed from masteries, you'll constantly be on your enemies back, slowing them nonstop with creeping death and your other abilities. If an enemy gets away with low life cast ignite andChildren of the Grave. Don't forget this also slows the enemy since Children of the Grave deals magic damage.