This is a late game build. Morde however is a snowballin character that levels fast and gets money really quick. He is quite the harasser if you know how to play him. Most everyone out there tries to build him pure ap with a little health. This works if your not taking alot of damage. If you are going against major ad(usually 2 or more) buy a thornmail. If not go with the Rabadon's Deathcap. The idea behind Morde is to harass til you get the enemy low enough then pop your ult and jump in. This build focuses on stacked items ie. Leviathan and Majai's Soulstealer. Stats for these items once stacked are:

Majai's
+20 Ability Power UNIQUE Passive: Your champion gains 8 Ability Power per stack, receiving 2 stacks for a kill or 1 stack for an assist (stacks up to 20). You lose a third of your stacks on death. At 20 stacks, your champion gains 15% Cooldown Reduction.
This equals 20+(20*8)=180ap and a 15% Cooldown Reduction.

Leviathan
+180 Health UNIQUE Passive: Your champion gains 32 Health per stack, receiving 2 stacks for a kill or 1 stack for an assist (stacks up to 20). You lose a third of your stacks on death. At 20 stacks, your champion takes 15% less damage.
This equals 180+(20*32)=820 Health and 15% less damage.

If you find you just need more health add a warmog's. This is only if you wish to be pure tank.

I use magic pen on most of my mages. It seems to be the best all around rune. I also use attack speed on morde just to increase his # of attacks when taking down a turret. Most of you know this takes forever.