I've been messing around with Mordekaiser a little bit the past week and I've come to realize he has great potential as a main tank or even better off-tank with great DPS, The majority of the items specialize in both Armor/Health/Magic resist perfect tanking stats all the way through. I've tried a straight DPS build on Mord but from my point of view he already has a decent amount of damage from just abilities alone, well over enough to kill most champions 1v1 or eve 2v1 - With this build In caster heavy games I've gone up to 280 magic resist with Creeping Death on and a great amount of armor as well. A few items are mix and match depending on what the lineup of the opposing team is.
The main reason for stacking both armor and magic resist allow him to take it from anything basically and having great health from Trinity's and Frozen mallet allowing him to still wreck some damage when you run in the fray with Creeping death dealing damage and giving you bonus armor/magic resist.
If the opposing team has very little magical damage do the following:
Switch out Mercury Treads with Ninja Tabi
Switch out Force of nature with either Frozen Heart or Thornmail - The aura from heart is great to reduce incoming physical damage but the Thornmail is really great if they have carries like Trist,Yi, Ashe the will destroy them self.
I really must insist on getting Aegis of the Legion, as I feel it compliments him very well giving you the bonus health, armor/magic resist and contributing the aura to your team as well.
This build is very strong even in a 1v1 hero situation unlike other tanks that will have problems if they get caught alone, Since you have his ult and ignite together they are basically dead if they are below 35-40% just from the Dot's alone.
I love this build just because even if you do have a tank I.E Rammus for example, You can toss your Creeping death on the main tank and give him more survivability along with an extra damage aoe. And you still have amazing dps for the staying power which is where a lot of the damage comes from, Once they see your items the will never target you first just because of your survivability and Guardian Angel, it would not be wise at least - so the whole time you have your damage ability's at your disposal.
Some of you might say the items are a little high end, but Mordekaiser is one of the best characters if not the best character in the game for farming, since it just takes a very small amount of his health, Your able to farm/harass all at the same time. With just a starting regrowth pendant, the 3 Quint runes you will have enough health regen to stay in combat forever early game to farm up your level and moneys.