This is how i play mordekaiser as a tank/carry.

i have Magic pen for reds because of mordekaiser's ultimate. The seals are health per level because he can be more of A tank/meat shield with it. I have magic resist runes for the same reason as the hp per level. the health quints are for a extra boost early game and the AP quints are for the advantage of his abiities.

i take 4 points in offense for the boosts of damage and AP per level. In defense i take 21 points for magic resist, armor, health bonus, AP, and attack speed which is really needed. In Utility i take 5 points for hp regen slightly more XP and improved ghost.

I like to take doran's shield for the health, armor and healh regen. My first complete item is sunfire cape because of HP reasons and armor since you are indeed a tank/meatshield (that happens to do a lot of damage :D) i get sorcerers boots for magic pen. Next item in my build is force of nature for the HP regen and magic resist. Abyssal scepter is for AP and magic resist and the reduce of other tanks magic resist. the gaurdian angel is optional but when i use gaurdian angel as mordekaiser i cant die.

i level up my Siphon of Destruction at level 1 and keep it there and from there max out your Mace of Spades while getting a level in Creeping Death at level 4 and in very late game
AND OF COURSE MAX OUT YOUR Children of the Grave because it is your ultimate and will get you kills and be able to tank turrets and sometimes even save your live because it gives you life steal while using it.

ok normally i take ghost] for the extra speed but in case you dont want that you can use [[exhaust since mordekaiser has no CC at all. but i think Ignite is a must for morde to make sure people dont get away with low health while using your Children of the Grave

OK every one i hope this guide helped you comment and rate what you think and tell me what i could change about it :)