This build is aimed at making Mordekaiser sustainable in 5v5 and 3v3 games. This build boosts his health early game letting him farm and gain levels more and it buffs him for mid game, so he can deal damage and just be a nuisance. Mordekaiser is mostly built on 3 things: armor/magic resistanc, health, and ability power. building on these in the right order will maximize his damage out put and sustainability. This makes Mordekaiser a little tanky and able to take tons of damage during team fights.

Starting out buy a regrowth pendant. This will let you stay farming while you still get a few hits by the enemy. Also because you are such a low level the health regen will be very beneficial. Also buy one health potion, and don't forget to use it when you need it.

Once you are about level seven and have gotten enough gold go back and buy either ninja tabi boots or mercury's treads(depends on if your facing ad or magic damage characters) If you have enough money start going for will of the ancients, as this will give you spell vamp and gain you health for the damage of your abilities.
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This should be about mid game and now is the time to get your ap up. If you have kept your farm up you should be able to now buy will of the ancients.

You should be later on in game and to enhance your farming ability I would get sunfire cape. Matched with your ability creeping death you almost always get last kills on minions. This is an optional item though and should not be gotten if you are not easily pushing on the enemy turrets.

Your next item on your list should be force of nature, but if you seem to be fighting against annoying ad characters then go for randuins omen. both will give you resistance and health regen. If you build into randuins omen sell your regrowth pendant and first buy warden's mail and then whatever else you can buy.
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This is the all up to you from here now... I suggest going for Rylia's Crystal scepter or Rabadon's death cap. If you want to have fun try going for zhonyas hourglass or a gaurdian angel (When you are resurrected by gaurdian angel and are in the middle of a team fight immediately use your abilities to get your shield up and hopefully get you a kill before you go back to base.

Remember when buying for items focus only on health/health regen, armor/mag. resist, and ap.

For masteries try to build his ability power, cool downs, health, and armor and magic resist.

Though I don't pay attention to runes that much, build what fits your needs and just keep it simple as ap, health, and armor (like masteriess)

Here is where you can easily go wrong with Mordekaiser, you must know when to use your abilities and what will get the most out of them. In the beginning get your siphon of destruction. You should be solo mid so you should use siphon of destruction to get lasts hits. Upon hitting level two get mace of spades. Use this in conjuncture with siphon of destruction to destroy minion waves. Once you get creeping death you can start to really farm. If the other champion isn't poking place it on your self lead with siphon of destruction get in the middle of the minions and use mace of spades before getting out. Be careful not to do this when you think you might be getting ganked. If you have a pokey champion just place creeping death on a minion and while they fight they will get the last hit for you. Upon reaching level six you can be more aggressive. While facing a champion use siphon of destruction while they are behind their minions, making sure it hits them. Place creeping death on him walk up to them, you should have a pretty good shield and they should be running away so turn and hit minions with mace of spade try to get a hit on the enemy chamion when siphon of destruction cools down use the combination of ignite then children of the grave and hopefully finishing them off with ghost and mace of spades for a final blowing kill and a pet ghost. Be sure when doing this though that you can catch them before entering their turret and that they have no tricks up their sleeve like ryze or morgana's cage. This offensive attack usually works better on melee fighters. In large team fights try to stay back and don't initiate the fight. Once the fight is initiated put creeping death on your self or another champion, lead with siphon of destruction, mace of spade, then focus on one champion hitting them with ignite and children of the grave. Back out of the fight and let your ghost do the damage. If you've got a champion on the run use ghost, mace of spades and siphon of destruction (only do this if you have one or more champions helping you.) Remember in fights that you can use children of the grave to gain you health so place it on the enemy with the most maximum health and and stay back while you gain health.

Because Mordekaiser is such a great farmer you can takeout a whole minion wave while their team is focusing on pushing mid. After destroying the minion wave go mid to help. After you manage mid go to the next lane push a wave of minions and continue pushing mid, do this at about level 10+ and only if someone will cover mid for you.

Go for ghost and ignite, or flash and ignite. I choose ghost and ignite because they are on the same mastery and because flash won't help mordekaiser get away because he is so slow. Heal may look like a good spell but if you build Mordekaiser right you will never need it, besides you will usually die even if you try to save yourself with it.

Pros: able to put out a lot of damage with little damage taken. Sustainable in team fights. Extremely good minion farmer and pusher. Can take down turrets and minions at the same time. Shield blocks most incoming damage. Great ultimate that can turn tables in team fights.

Cons: Weak when being ganked. Useless without abilites. Must have decent team members. Not likely to get the kill. Slow and is ultimately dead after being stunned or slowed.

So when building Mordekaiser get small health regen, armor/mag. resist, ability power. Then large amounts of health regen, armor/mag. resist, ability power. This will be the easiest way to remember it. Remember to always adapt to the fight though so you may want to change the order depending on who you're fighting against. Remember that his ulti and ignite make a deadly combo and that his ulti gives him health. Keep trying new stuff with him and find out what works best for you, but hopefully this build will give you some guidelines on mordekaisers strengths and weaknesses and how you can maximize playing as him.

Please comment if there is anything is missing or if there is something you would like to see on this guide.
thank you!