OK so I play tanks like dps champs, all my tanks carry, especially Morde. The items i get are different then what you might expect. I've only met a couple people who can play him like me and actually carry, if you can great. if not, please don't leave down-rate this because it "doesnt work." Because it does, just not for all.

SO i just redid my runes. the magic pen reds just to increase the damage of his spell. Hp regen per level yellows (obviously helpful since his abilities cost health). and cool down blues because when you need it, a little more shield can save your life.

I get the masteries of an AD champ because i build slightly tank-ish but with the masteries it gives me enough killing power early game to build quicker.

the first item for me is situational, either i start building a phage, or if they have a champ that you need to poke at for awhile before you can kill him, or i start warmogs, but most of the time i get phage, the little health and AD just make you hit a little harder and able to take some pokes. A lot of Mordes get Sorc boots for the magic pen but i don't like it, i like being able to chase down (or run away from) champs. then i slowly build a hextech. i start with the revolver first because the AP may kill you an ult kill that you otherwise might not have gotten, plus the spell vamp reduces the cost of his spells (meaning he heals the cost if enough damage is done). In regards to FoN, you can get it earlier if you are having problems with no health because of spell spamming. I think his spells are like the "mana *****" champs, you just have to learn to manage, find a balance between keeping your shield up and spamming. Recently i found that a PD let me get more kills with the AS + the movement speed, you may choose to get something else (if the game is still going) like and AP item.

Just like everyone else, i bring Ghost and ignite. Ghost to chase or run away, and ignite because combined with his ult, you almost always get that last little bit of health down and get your pet ghost.

SO, like Temzilla said below, Siphon is amazing. Use it to hit a whole creep wave. Although i disagree with him on maxing it out so early, i like having a decent level in Q because if you are lucky enough to have someone try and 1v1 you, your Q will HURT. The shroud is helpful, but i don't particularly use it all that much. Although one nice thing about it is, if you need just a little help in tanking a turret, wait till a creep wave gets there, Siphon them, then put the shroud on and stand in the middle of them while hitting the turret. Siphon will get high initial shield, and then Shroud keeps it up enough to help keep you alive.