Minions = Feed
Intro:
This is a short guide on how I play Mordekaiser as a very heavy AP carry.
Runes:
Morde is weaker than most other AP carries early game due to the costs of your abilities and your foes cc, this is why I like to use these runes to somewhat counter this.
Masteries:
Basic AP carry masteries.
Items:
To make this clear I do not always start with boots and 3 pots, depending on your foe you can start with a health pendant and 1 pot, and build it into a philo after your hextech revolver and boots.
The core build is DeathCap and Lichbane which make Mordekasier extremely OP depending how fast you can acquire these items.
Pros / Cons
+ I would say he is the best minion farmer in the game.
+ Epic pushing ability
+ you can live through crazy situations
+ Sheild = OP
+ Can carry and turn around games

- No cc at all
- Early game is difficult
- Susceptible to ganks

Summoner Spells
Okay I may be criticised about this but heal and ignite I believe are the best for morde in most situations, this is because your ult goes hand in hand with ignite and with heal you are able to do one of the best heal baits in the game. You can use ignite flash but usually if you don't flash over a wall you can be cced and caught pretty easy.

Play style (tips) :
Okay with Morde you need to change your play style completely. You need to be aware of the jungler and other ganks because it is nealry impossible to not push with morde, so you need to
use this to your advantage. After to demolish a minion wave you should either take there wraiths or yours, but if it is to dangerous just turret hug and watch other lanes. If you are getting ganked a lot place some wards down to negate this because you will need to catch up and wards do help. i play middle with morde because you counter most AP because they waste all there mana trying to take down your shield. If you see a lane pushed with one of the enemy champs on low HP go and gank, most of the time you will end up with a double kill. Morde can get easily counted by Leblanc if she is playing aggressive early game, a good way to stop this is to turret hug through laning phase and ganking top and bot.
Team Fights (tips) :
Okay your only goal is to get there ad carry and use your ult on them, this will make you near invincible because each hit that soul does will give you 30% as a shield (when it hits champs). If you are about to die just ult the person with the best HP/MR ratio because this will give you more HP to keep on fighting. Don't waste the soul, if you are near a tower or dragon make it at it and take it. People underestimate the power of mordes ult when you have an AD carry so use this to your advantage by baiting them while your soul is doing work.

Good Luck with LoL and if you need any help just talk to me in game, IGN - Seppi is awesome
(first guide, go easy :D)