I want to start off by saying you typically want to only play this build if your team has a tank already set in stone... otherwise this build may not be suitable since you will most likely be targeted. If you aren't targeted then enjoy the multi-kills.

Mordekaiser can be a pain to get out of a lane. If you're forced into a solo lane and doing a 1v2 just hug your turret and last hit the best you can. Don't forget to use your skills to hit multiple minions to keep your shield up. If you're in a lane with a ranged champion, play very aggressive. If you're with a melee you may have to play somewhat passive. When needed to go back try to grab your Ruby Crystal and Boots of Speed. From there on get your Warmog's as soon as possible as it will help you with your survivability and make you a nuisance to kill. Ionian boots should be followed up. These are your two core items aside from your Hextech Gunblade's. (HTGB)

All I have to say here is if you can offer your team a hand, don't get lost into your farming. Always help out, gank middle, your opposite lane if you just recalled, etc. Don't forget the basics. Make sure to only use your Children of the Grave and Ignite on their carry/your gank target when they're around half health. Also here is your opportunity to not only go on a ganking spree, but it also a great time to run through lanes and help let your team go back, heal, and shop. Your health regeneration rate is going to be ridiculous and annoying for the enemy team at this point in the game.

One note before going on to the next section, I recommend buying Hextech Revolver's out of any of the pre-required items for the Hextech Gunblade's. Simple answer : Spell Vamp

Alright, so the first Hextech Gunblade (HTGB) may have been a pain in the *** to get, but once you get passed the second one you are going to start seeing some great results. Mace of Spades for 800dmg single targets, 500dmg multiple targets, Siphon of Destruction doing rougly 500dmg, and your shield ticks for 150dmg. Again don't forget the basics. Basics as in, you KNOW where a Teemo is stealth (hint look at your shield bar when you use abilities) just simple things like that. Don't forget to try and position yourself to hit the most amount of targets as you can in the middle of team fights. It will help build your shield and keep your health up. Always ghost their carry and ignite them unless it's a Mundo.

Pro's:
-Great at getting in there and dealing a lot of damage unnoticed.
-Very hard to take out
-Can draw attention and can walk out alive
-Take out turrets very fast (200dmg swings or more)
Con's:
-Offers no Crowd Control (CC)