I am a very moderate player meaning that I am not the best but I try my hardest. This is my first build that I think works mostly. I'm not one to be saying this build is absolutely amazing I'm just saying I think it works and you can try it if you want and please leave comments on how it worked for you good or bad I like the criticism or compliments.

I think that greater marks of attack speed and attack damage are perfectly viable that way you still deal a decent amount of damage when you are farming minions.

I also think that the greater glyphs of magic penetration help especially in the beginning of a match because you will have a good amount of magic penetration to deal more damage on your enemies.

Plus the greater seals of scaling ap give you a significant amount of ap to start the round and they will benefit you later after grabbing the recommended items.

Lastly with all of my thought I thought that greater quintessences of scaling ap are very viable because you need ap for mordekaiser to deal excellent damage from beginning to end

See since this is my first build I may have messed up a little bit so look at the start items the boots of speed and amplifying tome the amplifying tome can be switched out for 5 health potions for sustain in lane if that is your type of gameplay. I automatically start building lich bane because of the movement speed bonus and the ability power and it has an excellent passive for late game and early game i think.
Next I try to build a Rylai's crystal scepter for ap and health, After that I build a hextech gunblade for the spell vamp and life steal that way when I used my abilities on a large group of people I actually gain life.
Late game I like my sorcerer's shoes for the extra magic penetration, I also build Nashor's tooth for it's cool down reduction and it's damaging passive, plus the ap and as

Now depending on your match you will either want to build out of three items 1 is listed on the items menu and the other aren't now the 3 items are deathfire grasp for its active ability and ap plus cdr, next item is rabadon's death cap because of its ability power and it's ability power passive, Lastly maybe you want rod of ages which isn't bad maybe you wanted the extra health.

Now I went with a 26/4/0 because the offense tree is awesome for mordekaiser's damage output is what I believe and the 4 in the defense tree were for health as you may know. I don't have complete reasoning for the masteries except I like my damage output really high in early game and late so if that's a crime then that's crazy.

I think mordekaiser is a very unique champion maybe because when I first played he was the one I saw first out of the rest and he instantly made me love the game
But his skills being Passive: Iron Man, Q: Mace of Spades, E: Creeping Death, W: Siphon of Destruction, and R/Ult: Children of the Grave
Q: Mace of Spades is quite useful I makes a basic attack deal more damage and hit more than one person but if your target is isolated then well it deals amazing damage
E: Creeping Death is also great because it his AoE skill plus it is a shield that you can put on yourself or an allied champion/ minion
W: Siphon of Destruction is a great skill because in early game it does excellent poke and I believe that you can harass opponents in lane
R: Children of the Grave is a totally bad *** skill this skill allows you too drain an enemy champion of their health dealing great damage plus if you kill that champion while they are cursed by your skill then their spirit rises from the dead to help you.

For summoner spells I generally take Flash for some type of escape method, Ignite for extra damage with children of the grave on, or Ghost but only ghost if you have already used flash or are saving flash or if you need to chase someone