Reds - Magic Pen, giving you 27 magic pen with Guise and these, most champions base is 30 so you will be dealing close to true damage to them, 47 with sorc shoes, if they have built some magic resist you won't be dealing true damage to tanks, but you will be dealing more damage because morde benefits more from magic pen then AP because his skills have a relatively low AP ratio

Yellows - Dodge, to give you a base 8% dodge chance so you can trigger your masteries to run away! and dodge is very useful more so late game when tryndamere comes up to you and wants to hit you for 1000.

Blues - I personally like cooldown reduction and getting boots of lucidity, so i have about 28% which is decent, but magic resist per level is good too, especially if you feel you don't you'll need sorc shoes.

Quints - I use two Health quints so early game combined with masteries you have +100hp, giving you the edge on your oppenent so you can buy a regrowth pendent instead of a dorans sheild.

and one Move speed quint, to give me a base of 320 move speed so i can outrun slow moving champions, and keep up with fast ones because morde has no CC built in him so he can be easily juked, the move speed edge from this quint is needed.

9/21/0

Archaic knowledge will boost your damage against people with magic resist items.

21 in defence because morde is looked upon as the soak of the team, and the points are wasted in offence and not as important in utility, the move speed and cooldown reductions are needed but the survivability is much more usefull.

Ghost + Ignite

Ghost for running chasing, you know the drill, morde has nothing to catch up with his enemies naturally, this will keep you in range or let you run away

Ignite is mandatory to help guarantee your ghost.

Good Alternatives
Exhaust - Especially for mid lane, helps you get first blood and what not

Flash - A movement ability that morde needs but not as much as ghost, you can flash to catch up but then two seconds later get outrun again and you can flash out but they can flash with you slow you or blink to you and all that, its only good if you flash through a wall. With ghost you can be slowed or blinked to, but you will still run faster then them after which is in my opinion better for morde.

Regrowth Pendant is a must for me, morde has some troubles dealing too make damage to himself early game by using his abilities, one regrowth pendent basically negates that and gives you some sweet regen to out lane your oppenent and force them back

Boots of speed to get tier one boots cheap, needed

Hunting Guise - Great early game item, help your HP pool early game so you don't have to rush rylies and can focus on resistances, good AP boost and much needed Magic Pen, and its cheap as, sold late game though

Revolver - Some Spell Vamp, pretty good on morde, this can heal you while you keep your sheild up giving you incredible staying power, and harder to kill while getting a good boost in damage.

Boots - Get what you feel is right for the situation, Mercuries treads, and Ninja tabi are needed if they have to much CC or their AD carry is wreaking you, and the Tabi will give you 20% dodge chance so your dodge speed boost will activate alot more.

Resistances - Based on enemy comp, and build the magic resist into FoN and you already have a regrowth pendant, this will heal you so much and with your spell vamp, you will hardly have to go back even after teamfights

CC - You can choose Mallet or Rylies, but i prefer Mallet, because it gives a 5% bigger slow and it applies to your ghost, which is so OP, if you manage to get a AD ranged from their team
it does so much damage anyway, and slows them for 30% it basically does more damage than you and they can't run away, and they both give a huge hp bonus which is crucial after one of them,

get a gunblade to like DOUBLE your damage, and it can be used as your ranged slow, for 50% which is AMAZING this is another reason why i get mallet first because i get gunblade for the ranged slow after

Then Rylies, having both mallet and rylies is really good on morde, with your ult it permanatly slows them so you can catch up and kill them or just slow for your team, and will boost your damage and health by alot again

Gaurdian Angel - Because you're not unkillable enough with your around > 50% resistances and huge amount of hp and constant sheild, so you can really run in and 1v5 they waste all their cooldowns and your team can EAT THEM ALIVE then you get up and PUSH.

I haven't said anything about the gameplay and tips and tricks, but this is my build for morde, hope you like it try it out! and if you really want tips, tricks and laning and general gameplay, give me a comment and i'll get back to it xD Thanks for reading!