Wow, u got enuf HP? dont u need some dmg besides infin. i dont know i dont use morde but i think he needs def,dmg,and health. and from what i uderstand about him is that he acually does most of his dmg as magic dmg sorc boots might help. look in to that cus i wont i just use jax, (yea i know he is op) but gl, if u win useing this build more power to ya.

ICE, out

uhhh...i dont even know where to begin. IMO cleanse is a must on just about every champion i play. on top of that, ignite works like a charm when used with morde's ulti. Also, when i tank, i like to have tank masteries, that might just be me though. I also like to have tank items for tanking. when i play morde, first thing i get is regrowth pendant and immediately start working on my warmongers. The best thing about morde is his ability to farm. this works great with warmongers.

My items for morde:
1. Warmongers
2. Mercs or ninja
3. Rylai's or frozen mallet
4. Force of Nature
5. Randuin's
6. Visage, warmongers, phantom (if they have crazy dps), thornmail, banshee, any support item.

Also, your runes kinda suck. As morde i prefer MR glyphs, dodge seals, and after that i really dont have an opinion cause im poor and cant afford runes. i currently use magic/armor pen quints and marks cause thats all i have.
Quints - MS, dodge, HP (if you have trouble early game)
Marks - MR

Basically, if i choose a tank, i tank, theres nothing else to it. I dont bother getting dps, all i do is survive everything they throw at me, slow them down if possible, and buff my teammates if possible.

P.S. i suggest 9-21-0 for masteries. get the magic pen in offense.