WARNING! - if you dont remember to use the items in the build, this is a REALLY bad build!!
This is my first build on mobafire. this build is a pure mordekaiser burst damage built, with this build noone can beat you in a 1v1 fight(unless they have like 300 magic resist, which ****s any morde build^^)

the magic penetration is ofc for the most possible damage from your spells in the early game, so you can get that delicious feeding from the enemy team:)
health is cause morde isnt the most tanky champion unless you build him as one.
ability power is just always awesome for killing stuff:)

i chose these masteries to help myself survive, and then also give myself that extra boost to my AP. and that extra tankyness could very well save you.
the 9 points in offence is for the extra AP and then the 15% spell penetration which is a great help in mid to late game against tanks.

i pick deathfire grasp and hextech gunblade to make myself deal a ton of single target damage, with ignite deathfire grasp hextech gunblade and your ultimate 1 person is nearly always dead, if not you just chase them and use your other spells.
other items are for making morde a bit more tanky instead of just getting facerolled by ad carries.

Deathfire grasp+Ultimate+Hextech Gunblade+Ignite+(standing alone)Q(standing near others)E
farming minions you aim your E against the most possible minions so you get maximum spell vamp and shield from them, and then you finish off the rest with your Q if you have enough AP.
when there is alot of minions i usually use my W on myself and then do the same thing as i wrote before.

Flash is always handy cause you can escape and catch up to people with it, and ignite is for the most possible burst damage.you could replace flash with ghost, which would probably be better for mordekaiser, but i personally prefer flash because of the juking potential and for ganks and stuff.

that nearly no other build can deal as much single target damage as this build can if you remember the items.
And that you do not lose any health from using your spells on minions if you have full build because of the spell vamp of your items.

hope you liked the build, please give it a few tries before you downvote/thumbs up the build, cause it might not work in the first game that you try it
Good Luck and Have Fun annihilating in 1v1 fights!:D