Introduction
-I'm a rank 30 LOL player, just like many of you. Some of you may be looking for a way on how to be better at a champ, in this case Mordekaiser, and most are just looking for a new way to play him. This build covers runes, some masteries, definitely the item builds, and some key strategies. Don't bother looking up my old Jax guide, that one's outdated and a bit incorrect.
This guide focuses on you being this Tanky AP Burster. You will deal enough damage to destroy a carry in 2 seconds flat once this build is completed.
Runes And Masteries And Summoner Spells
-Not the most important, but it's always key to a successful guide. I recommend getting:
9x Greater (Magic Pen.) Marks
9x Greater (HP5) Seals
9x Greater (CDR For Top or Magic Resist For Mid) Glyphs
9x Greater (Pretty Sure There's Magic Pen.) Quintessence
Why these runes? First of all, Mordekaiser's AP ratio/scaling is pretty low. 0.4, 0.6, so forth, that'ys why you should mainly include these marks. The HP5 seals are for staying in the lane, and spamming your abilities more often. Mordekaiser's abilities use health, so it's best to get these with your build. The CDR is for top lane, since you won't have much CDR at the end of the build and it's very helpful for using your ultimate more often. The Magic Resist is great for mid-ing, since you'll be more tougher against harass and you'll be more tanky/durable. The quintessences have the same reason as the marks.
-Masteries don't provide the GREATEST boost, but this is just in my own opinion.
I recommend getting 14/16/0. These focus on being more durable in lane, yet improving your farming and offensive magic abilities.
Offense:
3 Brute Strength
4 Mental Force
2 Butcher OR n your two favorite summoner spell masteries
4 Sorcery
1 Arcane Knowledge
Defense:
3 Resistance
3 Hardiness
4 Durability
3 Vigor
1 Veteran's Scars
2 Indomitable
Summoner Spells
-Almost forgot.
My favorite two spells are :
Teleport And Flash
Some other ones you could go with are the classic Flash and Ignite, or Heal and Flash. I prefer heal over teleport, since they both have a similar cooldown but teleport is much more tactical.
Skill Order
-This'll be short and painless.
Your main priorities should be your E and Q, with more emphasis on the E ability.
Order is as follows:
E>Q>W>E>Q>R>E>Q>E>E>R>Q>Q>W>W>R>W>W
This skill order focuses on harassing with your E more effectively, and using your Q to farm more efficiently. The W is just to piss off your enemies when they try to farm minions. 
Item Build\
-The fun part. The VERY fun part.
Your starting item depends on your lane.
-Doran's Shield if you're solo top.
-Regrowth Pendant/HP x 1 if you're solo mid.
Solo Top's tend to be more aggressive and physical, so the doran's is an obvious choice against them. Durability + Health Regen FTW.
Meanwhile at mid lane, the regrowth pendant allows you to stay and farm much more.
This merges into one build BTW!
Top laners should then do this:
Get like 1k, go back and buy boots and regrowth pendant.
Buy another regrowth pendant, sell dorans, buy negatron cloak.
Build Force of nature.
Mid laners should do this:
Get 1k, get regrowth pendant again and shoes.
get another pendant. and negatron cloak.
buy force of nature.
Then it merges.
Upgrade shoes into sorceror's shoes.
Get a Giant's Belt, Tome, Or Wand depending on your wealth.
Build a Rylai's Scepter.
Get a hextech revolver.
Build hextech gunblade, not a will.
Buy a sheen.
Build a lich bane.
get an abyssal scepter.
Your COMPLETED build should look like this.
Sorceror's Shoes, Force Of Nature, Rylai's Crystal Scepter, Hextech Gunblade, Lich Bane, Abyssal Scepter
I can explain every reason of why these items are here.
1.) The shoes give magic penetration. DUH
2.) Force Of Nature allows you to stay in lane, and be VERY resilient to other AP casters.
3.) Since Mordekaiser doesn't have any natural CC, and is also semi-squishy, this should be a must. The AP is quite nice too.
4.) Hextech Gunblade is better for Mordekaiser than WotA. Hextech has about the same amount of AP, it gives life steal which is an added bonus, and the attack damage scales with his Q, as it has a +1 Bonus AD Ratio. The active is veeeeeery useful, since again, he doesn't have natural CC.
5.) Lich Bane. Does 1000 MD every 3-4 seconds sound good enough for you, when you use Mord's Q? Hmm? Basically, it makes the mace turn into ACE.
6.) Abyssal Scepter. Mordekaiser will go into melee range very often, so the aura will be used. He gets bonus magic resist, to make him even more invincible against casters, and the AP is decent. With this, a normal carry's magic resist will go down to 0 or below.
Why doesn't this build have a DfG? I don't feel like using it, basically. The CDR and the active is very nice, and you can personally replace it with any of these items if you feel like it.
KEY TIPS
-Ooh. This is a bit complicated, so let me sum it up a bit.
>Focus on harassing with your E, yet using it to farm minions. It's practically the only harass you can instantly deal, so use it.
>Use your Q to last hit. it's great at clearing minion waves too.
>Cast your W occasionally on a minion right behind the leading minion in a wave. It'll make farming a pain in the arse if they farm next to the minion, as the W does have a +1 AP ratio when in there for the maximum time. Ouch.
>Cast your R when you want to bait someone. You get healed for it, and I think the passive is applied. Cast it on squishy carries for maximum benefit, since they have more of an autoattack usefulness than most ghouls. Also applies for Yorick (wut?)
>Do not be overly offensive. you may be outbursted if you don't have your shield and you spammed your abilities on their cd. That's a no-no.
>Play on the safe side. Cast your R on the focused enemy, and stay at a range until they focus someone else.
>Achieve awesomeness by fully embracing the means of his skin. PENTAKILL.