Hello everybody, this is my first build :D. I'm writing as is, so if I end up saying stuff from other guides, I'm sorry :(. Mordekaiser is supposed to be played as a beefy caster, dealing massive AoE damage while taking almost none due to his shield. He shouldn't tank, since he has no CC and can't initiate. If you build full tank, people will go to great lengths to not attack you, since they will do absolutely no damage. If you build full AP, people focus you first and you die. Very fast. This builds gives you a lot of armor/MR, a lot of CDR, and a decent amount of AP.

If you want more detailed information on Mordekaiser, I would recommend reading Unforgiven's Guide

Pros:
Amazing AoE damage
Exceptionally hard to kill unless focused
Amazing farmer/pusher
Can survive a lot of 1v1 and 2v1 situations
Insane ulti
Insane laning phase

Cons:
ABSOLUTELY NO CC
Kinda have to be careful with how much you use your abilities early game.
Very hard to fit on a team (in Summoner's Rift) (especially ranked)

Morde is a beefy caster, and even though you would think that defensive masteries would work a lot better due to his shield working with armor and MR, you get much better stats from the utility tree (CDR FTW)

Magic Pen marks because almost all of your damage is magic, and the more damage you do the more shield you get.

HP-Level seals, flat health isn't that good, HP regen would probably work for early game dominance, but idk. I prefer having more health late game.

Flat CDR glyphs are wonderful for morde.

I choose 1 magic pen quint to make it so you have 9.7 (rounded to 10) magic pen. With Abyssal Scepter, MOST champions will have 10 magic resist (unless they build MR, in which case you will eventually build void staff) This makes your spells do full damage, thus making your shield stronger.

2 AP quints since this build is for beefy AP morde, and early game damage helps out greatly.

Ghost: Lets you catch up to fleeing enemies, then they can't escape b/c you have Rylai's. Also lets you run away (if the moment ever occurs).
Ignite: Needed. Helps get early kills, helps secure kills with your ultimate.
I wouldn't recommend any other spells.

Iron Man is your passive. Self explanatory, benefits from armor and MR, it's awesome. It's what makes Mordekaiser unique.

Siphon of Destruction is your main ability, leveling it first is a must. Spam it in team fights, farm with it, check bushes with it, harass with it, touch little children with it. Try to hit as many people as possible with it, and during laning phase try to harass people while hitting their minions for additional shield gain. Don't try to harass with it until level 3, just last hit with it until then.

Children of the Grave is one of the best ultimates in the game (if you can use it right, it is THE best ultimate) so you always want to take it when available. Ideally you want to use it on an AD carry with low health, or someone who deals good auto attack damage, so you turn a 5v5 into a 6v4. Always combo it with ignite if you can. If someone on their team is very low health, is getting away, and you're sure that your ulti will kill them, go for it. Better to have a ghost who does some damage, than to let one of them get away and waiting to use your ult to the very end to kill an elusive AD carry.

Don't use your ulti on a person with Quicksilver Sash, anyone with Guardian Angel, a spell shield of any kind (Nocturne, a person with Morgana's black shield, Sivir, anyone with Banshee's Veil), anyone with Kayle's or Zilean's ulti buff, Anivia if her passive is available, Trynd (wait until his ulti buff is gone) alistar when his ulti is up, Blitzcrank when his passive is available, Mundo if his ulti is ready, anyone with a heal that doesn't have that long of a cooldown (you have to judge for yourself, if you know Soraka is low health, is getting away, and had just used her heal go for it), Nasus or Renekton (if they have their ultis available) Poppy (when her ult is up) AND your ult will do half damage to her thanks to her passive, anyone with a shield that won't die to the initial damage. These cover most situations, but learning when to use your ult just comes with practice.

Creeping Death deals amazing damage, gives you (or your carry, or anyone who is being focused) decent armor and magic resist, and lets you farm like a bawse. You can also use it on minions (you can put it on one of your melee minions to generate shield) wards, etc.
This is why you don't get sunfire on morde, because having CDR and some ap with this skill is essentially a free sunfire passive.

Mace of Spades deals some OK damage, but is last priority. Late game this will do some pretty good damage, and will devastate single targets. And it has a relatively low cooldown. Also resets auto attack timer, so you can use this to your advantage while last hitting and taking down turrets. Cool beans bro.

Early game, use your Siphon to last hit minions. Once you reach around level 5 you can use your Siphon, Creeping Death, and Mace of Spades to clear minion waves rather quickly. Just remember to get the last hit, no point in destroying a whole minion wave and getting no cs.
Nothing much to say here since it's Mordekaiser...

FIRST OF ALL: You want a solo lane (if you have a jungler) or a lane with a support who doesn't need that many creeps (always go solo top unless you have no jungler) since Morde is a creep killing machine, and needs the gold. Don't go mid unless for some reason you don't have a carry or an AP nuke or anything that normally would go mid.

On a second note, what makes mordekaiser unstoppable is his shield. His shield benefits from armor and magic resistance, which makes stacking hp ****ing stupid, and makes stacking armor/MR and HP regen awesome. Also, the more frequent you can use your abilities (where CDR comes in) the more you can charge up your shield. A mordekaiser with little CDR but a **** load of AP (well more AP than this build provides) generates almost a full shield with all of his abilities (besides Mace of Spades) but less frequently. With this build, you generate your shield almost to full (almost as much as a full ap build) but more frequently. This makes you bat**** crazy and requires most of their team focusing you to bring you down.

Start off with a regrowth pendant and a health potion. This is pretty standard for Morde, I shouldn't have to explain why. Farm enough for boots of speed, another regrowth pendant, an amplifying tome, and 2 sight wards. More HP regen = more HP to use for your skills. Amplifying Tome gives you more early game AP, it isn't needed, but I find it helpful. Boots are a must, and I shouldn't have to explain why. Why 2 wards? You need to farm your *** off early game, and a gank will set you back. Knowing that you can push the enemy back and farm creeps safely is worth 150 gold. 6 minutes should be enough time for you to farm enough creeps for your Force of Nature (which gives you all the HP regen you need to spam your abilities)

Now, why I chose boots of lucidity over sorcerer's boots is for CDR. Almost every item that does have CDR also has mana or mana regen. I previously explained that you're going to build Abyssal Scepter (unless their whole team is AD in which case you would build Haunting Guise) so people have reduced magic resistance and generally puts them at 10 MR (which you get rid of due to runes)

Up until mid game has started, whenever you go back buy a couple wards so you can make sure you don't get ganked.

Rylais Crystal Scepter is the perfect item for Morde. He has no built in cc, but this item gives him 4 slows, gives him a lot of HP, and some decent AP. You need this item, no questions.

Now this is where items get tricky. Look at their team and see who is doing most of the damage.

If it's AD, start building Randuin's Omen. This gives CDR, HP, Armor, HP regen and even more slows. Woot. If none of them are building MR, then build Abyssal Scepter (explained previously) If 3 or more of them are building MR, disregard Abyssal Scepter and work on buying Void Staff. For your sixth item, you can either build Rabadon's Death Cap if you find that the enemy team isn't doing that much damage and if you could use a little more damage yourself. If you are still getting *********ed by their AD, build Zhonya's Hourglass. Armor and AP, and an awesome active.

If it's AP, start building Abyssal Scepter. This item is ****ing awesome and if you don't think so go DIAF. Abyssal and Force of Nature should give you enough magic resist. If the enemy team has any AD damage (Including Lich Bane) work on Randuin's Omen. The armor also helps with turret damage, which is nice. If they have absolutely no AD damage, aren't building MR, and you find yourself still taking a lot of damage, I would suggest buying Warmog's. At this point, stacking any more MR wouldn't help out as much as HP and HP regen would. Although Warmog's is quite expensive and takes forever to get the bonus hp and hp regen, you're ****ing Mordekaiser. You SHOULD be getting massive amounts of gold from farming and team fights, and killing creeps for the bonus health/HP regen isn't hard, since Morde farms creeps very fast. After Warmog's, buying Rabadon's Death Cap will greatly increase your damage. If 3 or more people are building MR on their team, work on buying a Void Staff, then Rabadon's Death Cap.

If their damage is split amongst physical and magic damage (which happens 90 percent of the time) buy a Heart of Gold, and a Warden's Mail. Don't upgrade it until you get Abyssal Scepter, since the gold per 5 from HoG is awesome, and upgrading Warden's to Randuin's Omen isn't as important as Abyssal Scepter's MR reduction. For your sixth item, either buy a Rabadon's Death Cap if you find you aren't taking that much damage from turrets and their AD. If you ARE taking considerable damage from their turrets/AD then buy Zhonya's Hourglass.

If you have any suggestions pertaining to items, don't hesitate to add a comment. Constructive criticism is always welcomed.

Don't initiate. You aren't the tank, you are a soak at best, but not a tank.
Once the fight has started, try to hit as many enemies as possible with your Siphon and Creeping Death. If you can try to focus their carries (or whoever is doing the most damage) and spam your Mace of Spades like a ***********er (You should be within melee range at all times) Use your ult on their low hp carry, or someone else. I explained how to use your ult previously.

Try the build out before you down vote. Sorry this guide isn't fancy, but idk how to add items or champion thingies >.> I probably will later on once I know how.