Originally Posted by Sauron
Runes: MPen, flat Armor, scaling MR, MPen quints
Items: Ionian, Randuin's, Rylai's, Void, Abyssal, Lich Bane
Strategy: Mordekaiser is a bottom laner and does not have a terribly strong start. Nonetheless, he can either go a flat MPen build and switch it out later, or if the laner is not particularly lethal to him then he can go the Ionian/Void route right off the bat. Leveling his E first is an obvious choice as it lets him harass and push at the same time, while Q versus W is more of a situational decision. If you're dealing with someone you frequently get close range with, take Q, or if they keep getting close to their minions, take Q and use the splash on it to hit them. Otherwise, take W. Morde doesn't do a whole lot other than farm and harass until he's level 9 or so, when he can start to actually make moves on the enemy laner.
And if you didn't know, Morde's Q converts Lich Bane to magic damage, which can be devastating if you decided to go Gunblade for some reason. Unfortunately both items are very expensive.
I feel its my duty almost to add to this or to give my perspective. So here it is, I'll give more details when I write a proper guide for this guy. I also feel writing a Tank Ahri section, but I am too tired atm.
9-21-0 or 21-9-0
MPen, flat Armor/scalinghp, scalingcdr/scaling MR, spellvamp/ms quints
huE->W->Q or Hue->2w (levels 2 and 4) ->Q->3W
Sorcs/Ionion, Frozen Heart/Thornmail, Void, Hextech Sweeper, Abyssal, Gunblade, Lich Bane
Mordekaiser has one of the weakest early games in the game and is very prone to being snowballed against. He has no innate esacape but his passive when used effectively gives him the highest sustained damage migration in the game. Unlike in SR rushing or getting any form of early spellvamp item such as Revolver or WotA ends up making you weaker rather than stronger, Revolver should be gotten early only vs other mages but you still need at least some spellvamp which is where the Quints kick in they are enough to negate your HP costs on medium sized waves. Your first goal is to get enough damage to be able to 1 shot the ranged minions with an E by level 9, this will allow you to safely stay at your tower and defend any push which is the best thing todo, leaving your tower to push is very risky for the aforementioned reasons. Your secondary goal is to make sure you can sustain your W for fights, this is your only protection against dying to CC (constant shield regen). If you are confident in not dying max Q after 2 points in W (a second point in W is a 58.3% damage and shield regen increase per cast and is required in order to hold an AD Malzahar, Yorrick or Jax Lane without dying). Final goal is 30% cdr, high ap and lichbane so everyone dies.
Don't question Frozen Heart or Hextech Sweeper. The first even though some gold is wasted on mana it is still your best in slot item if you need both armour and cdr (700g more than thornmail for the same armour but 20% cdr and a better passive). Hextech Sweeper is used mainly as a protection from ganks while also providing the stats you seek, the active and passive can both save your hide and setup kills, especially vs annoyances like Kassadin or Ezreal.