stacking up some spellvamp, HP, resistances, and a load of AP on morde is a force to be reckoned with. Remember that you have a passive that creates a shield for you based on the damage you do with your spells. so Cause havoc while building a defense and win your lane. The Spell Vamp will allow you to stand in mobs and essentially heal yourself while causing massive amounts of damage. HP is ALWAYS beneficial and having high HP and high Spell Vamp allows you to have a higher room to heal for. resistances will stop bursty champions from eliminating you since you will be causing a lot of problems for the other team. AP will allow your to DESTROY groups of champions since all of your abilities except your ult hit entire groups. And high AP will make your ult hit for over 60% of current health so in combination with your abilities and ignite it will be difficult to survive your wrath.

Marks of Magic Penetration
Seals of Armor
Glyphs of Scaling AP
Quints of AP

In Order
Start with Boots and Pots
Rush a Hextech Revolver
Finish Boots into Sorcerer's Shoes
Build a Rylai's Crystal Scepter
Build a Liandry's Torment
Finish Hextech Revolver into either a Will of the Ancients or a Hextech Revolver (personal preference I recommend the Will of the Ancients becaue it has more Ap and has an Aura to aid your allies)
Build a Rabadon's Death Cap
build a Zohnya's Hourglass
finally sell your Sorcerer's Shoes and buy a Spirit Visage to round out your build (be advised you can buy multiple items and be successful but I recommend the Spirit Visage because you lack Magic resist and extra health and increased spellvamp is highly beneficial for this play style)
other HIGHLY recommended items instead of Spirit Visage are...
Abyssal Scepter- (AP and Magic Resist while reducing the Magic Resist of champions around you. VERY good for Morde's play style since he is meant to be used in piles of enemies)
Warmog's Armor- (INSANE HP AND REGEN)
Guardian Angel- (Armor and Magic Resist and allows you to come back to life after being killed and continue to murder face)

LEVELS TO TAKE SKILLS AT
Q Mace of Spades- 13, 14, 15, 17, 18
W Creeping Death- 2, 4, 8, 10, 12
E Siphon of Death- 1, 3, 5, 7, 9
R Children of the Grave- 6, 11, 16

Your E is your primary poking ability and most commonly used, hence why I recommend maxing it out first and foremost. Your W is the Best Sustain you have with all the spell vamp you build and is insanity in groups of minions or champions. Your R or ULT is DEVESTATING and in combination with Ignite is very difficult to deal with. Your Q is good in groups as well but I just don't find it to be as helpful as the other abilities which I is why I ignore it till the end.

Flash
Ignite
NO QUESTIONS ASKED.
To finish champs, Flash in, Ignite, Ult

Being one of the few tanky AP carries in the game and having the shield as a passive you are able to bully enemies and push the lane. Use your E to destroy minion waves and keep your shield up. once you have the Hextech Revolver it will negate the HP cost from your spells. You will then be able to massacre minions with your W and E and poke at enemy champions willing to come in range of your E.