I update this build time by time with my experiences i make with it ( I not always build like this because its an sitiuational build maybe i will add some alternative builds later. Have fun with this build and feeld free to comment and testing it. If You catch Up a mistake comment on him plz i will change it as soon as I can. This includes double pousting cause i edit a lot on the build and im feeling like I could have lost the overwiev already! :P

Greetings

Items: I try to maximise movemt speed, healt rgeneration and magic resist.

Laning phase: trhoug early life regeneration want to minimize the times i have to get back to base. ( 2955/207*5 = 71seconds from 0 hp to full hp at full build)

Teamfights: Focusing the squishy dmg per second normal atackers and elminate them fast with ultimate/other skills. Keep near the enemys to maximise the damage of Creeping death and build your shield with it.

Item Purchase Order

Items Purchase order if u face a lot of harassing on your lane

EDIT:
I know that passive of Force of Nature is Unique.

This is not a guide about "How to play Mordekaiser?".
It is not THE itembuild that is the solution for everything.
But it is in Option depending on the situation. And I can tell you that it Works pretty well!

Pros:

Pretty well health(warmogs)
Very good movement speed (Boots of Swiftness, Force of Nature)
A ton of hp/5sec(Force of Nature, Warmogs) (at Full item build around 207 hp/5sec|41 hp/sec)
Very good resitence (Mdef(obvious) 78%reduced ,Pdef(Runes) 52%reduced(without (W)Skill :P))

Cons:

No bonus dmg(AP/AD)
feared of critical spammers (Yi/Tryndamere)
No disample through items
expensive (but it worths ;) )

In my most matches with this build i go back to base around 4-8 time, in 50 % of matches 6 times. Till u are level 9 and you have at least the 4 Regroth Pendant, you can go jungle and trust me you will get back to lane with a lot more life then you gone away from because ur shield will eat all dmg and your skill and melee atack is tough enough to beat them down. Its always also a possible decission to change lanes and defnd lonely towers from aztacking creeps. Just have in mind that you shoulndt leave your lane alone fo too long cause the enemys could rush your towers then.

Update:

I changed Heal with Cleanse because in mid to late game thx to the insane health regen i dindt nead it realy, but I profit well from eliminate the disables to build my shield with skill damage.

If you have a Jungler in ur team, its always a good Idea to learn Creaping Deathfirst wait till he Atacks Creeps in jungle first and cast Creeping Death on him. Then Return to lane. While this this fully fills Your shield and helps the jungler great to kill golem or other faster then normal and easier (more health). Works realy good with amumu jungler on golem :)

If you ever get to the point of Full Build, kick out Boots of Swiftness! Why? u get +32% extra movement speed (means 315 * 1.32 = 415,8), so u can put in an item u prefere. Maybe Trinity Blade or another Force of Nature ;).

But wait! Some last words:

I won't tell you, that you will always win with this build. Have this in mind if you vote me.
The realy efectness gets to this build if enemys make most damage as magic damage.
Have in mind that not only mages make magic damage.

Exapmle enemy team I played against with this build: Karma, Nunu, Amumu, Ashe, Zilean. Look at their description and summarice how many skills of them deal normal damage and how many deal magic. Ha Ha! I can tell you there where some funny situations till mid game :P
And before you can say that this is a horrible team take another look:

Karma(Burst mage), Nunu(jungler), Amumu (tank), Ashe(carry), Zilean(support). They had everything but not in a realy good combination (because of my item build :D ).

Another team where my buil worked good (13/2/13): Kassadin, Maokai, Twisted Fate, Vladimir, Cho Gath (After second force of Nature there where not ablye to get me down anymore and i keeped killing/ harassing them, thx to my good movement speed :) )