Introduction
Hi there summoners.
Ok, so this is my first LoL Champion guide here at Mobafire but it is definitely not the 1st or 100th time that I play 
Mordekaiser, so I'm not going to throw out opinions. I'm going to talk about my gameplay experience with him within the past year and what its the best for playing with him.
* I made this in a couple of hours, so it may not be completed, or grammatically correct. *
Items
For items I like to start 
Regrowth Pendant + 1 Healing Potion if going to take Mid-lane, or 
Doran's Shield if I'm going against an AD Carry on top or mid-lane.
Why no Boots and potions? Because potions are meant to keep you alive or sustained on your lane, not preventing you from dying from your own abilities. You should buy boots IF you're not so good avoiding skillshots from champions like 
Brand.
After the normal boots, you should buy 
Sorcerer's Shoes if you need to hit harder and do more damage, 
Hextech Revolver if you need more sustaining or Giant's Belt if you're too squishy (taking too much damage, that prevents you from farming or pushing your lane)
-Get an early 
Rylai's Crystal Scepter if you and your team need CC.
-Get a Will of the ancients if you're doing well.
Mid-game:
Now for the mid/late game you're going to get some serious ****. At this part of the game you will need to know which equipment fits better. Is it a Thornmail? Randuin? Guardian Angel?
Heavy AP: Get a 
Force of Nature.
Squishy ADs: Go get a 
Thornmail.
Offtank ADs: Grab a 
Randuin's Omen or 
Sunfire Cape.
Balanced Team: Shop for a 
Guardian Angel.
Late Game:
At this point you will need to know if your team needs a Tankier Mordekaiser or a Damaging Mordekaiser. You should get more Defensive Items if your team don't have a proper tank or can't survive the teamfights, otherwise go get a 
Rabadon's Deathcap + 
Void Staff, Rabadon's Deathcap + 
Lich Bane, 
Hextech Gunblade. You can also get a 
Madred's Bloodrazor which is pretty good xD since you already have decent M.Penetration from runes, masteries, and sorcery shoes which synergises well with Madred's Bloodrazor passive and Mordekaiser have a decent base Attack Speed.
You can, and you will get more defensive items, if there is no Tank in your team.
Gameplay
So here we are on the most important part of this guide where I'm going to point the most relevant aspects of Mordekaiser, give you tips and such.
Early game / Farming the lane:
So the reason you should start with HP regen is that you can get your HP back right back, after casting your spells. Getting boots means that you're actually helping your opponent by harassing yourself.
-Don't spam your abilities, keep the minion waves at the center of the lane.
-Use your E whenever you can to drop a few HP bars from your opponent.
-Keep your shield at 50% - 100% when you expect incoming harass.
-BUY WARDS, and keep yourself farming near the bush where you did put the ward. If you're top-laning, stay near to the upper bush.
-Retreat if your opponent gets TOO AGGRESSIVE or TOO CAUTIOUS. The're probably expecting they're jungler to gank you.
-Ask the jungler if you can farm his wolves or wraiths while he is at your base or ganking.
Mordekaiser can't win 1v1 situations in early game. In order to score kills we need to kite your opponent and harass him, till they drop below 40% HP.
-If you're going after an enemy you can start with your Q on the nearest minion from you opponent. Remember that your Mace of spades will hit the targets behind or at your side.
-If you're sure that your E » Q » E » Ignite is enough to burst your target go after him, if not, kite around a little more.
When you reach LvL 6, that will be another story. At mid-lane you can bait your opponent by over farming while he is going to call for a gank. Keep spaming your E on the minions and opposing champion. If the jungler comes in you have 3 choices:
-Run like a little kid
-Steal the mid-laner copy with your R and go after the Jungler
-Copy their jungler ( If he is low on HP ) and BLOW UP they're mid-laner
Mid-game:
Mid game is all about getting survivability items and adjusting your equipment according to the game and enemy team. Now with the 
Giant's Belt and Spell Vamp you're probably able to soak a lot of damage and still do decent damage, so you will be extremely annoying to get down. But be careful, you're not yet the best Dueler 'cuz your CDs are still a little bit high until 14-18.
Late-game:
Now, being tanky enough, you want to hit hard again so you're going to turn your back to defense and buy some Raw Ability Power items or Magic Penetration.
Your game style should be sticking onto a target and NEVER give up. Your spell vamp and shield should keep you alive.
Some important aspects plus Tips
-An OffTank Mordekaiser's purpose is to unleash FEAR your enemy team. You can't do a lot of damage and you don't have hard CC. But enemies will fear you because you're indestructible. YOU ARE BETTER than Dr.Mundo 'cuz you have AoE damage, Spell Vamp and decent HP Regen. And they know that you can steal their AD Carry in a matter of seconds.
-Mordekaiser doesn't benefit too much from AP, so investing in some M.Penetration can some times be a better option.
-Your main priority should be the Squishy targets 'cuz with a decent amount of magic penetration you will be dealing TRUE damage to low HPs.
-Unless your team lacks of a proper Tank, Morde should never initiate alone, because he plays an important Supportive/Damaging/Chaos role on the battlefield.
-Your teammates will eventually forget that you have 
Children of the Grave. Remind them to focus the AD carry or any other AD enemy. That way you will turn a 5v5 into a 6v4 situation
-Is their AD CARRY fed? EVEN BETTER. Enslave him with E»Q»E»R»Ignite and go grab a 1v5 pentakill.
Build Guide
Discussion (3)
Greater Quintessence of Ability Power, being unharassable in Mid-lane with flat MR 
Greater Glyph of Magic Resist, and survive the ganks with Flat Armor seals 
Greater Seal of Armor.
Summoner's Wrath
and M.Penetration
Arcane Knowledge
in Offense, HP Regen
Vigor
and CD reduction
Enlightenment
in Defense. "Why do you take 3 points in Atk Damage
Brute Force
?" Just because it is be easier to do "Last Hit". "Then why don't you take [[Butcher] in offense?" 'Cuz Morde's Q scales with AD.
Flash, 
Heal, 
Exhaust, 
Teleport, 
Cleanse (Draft mode - When you know that you're going against heavy CC). This will depend of the situations. Otherwise TAKE ghost and Ignite.
