Here is my build for Mordekaiser (my favourite champion) if you have something to say, notice, add just post a comment.
Thank you for reading.

For starting you need Amplifying Tome and a Health Potion so Mordekaiser can have some more initial AP.
You need Sorcerer's Shoes (recomended) for magic pen.
You need Liandry's Torment for some farming or good harassing with Siphon of Destruction.
Get Will of the Ancients or Hextech Gunblade (recomended) for spelvamp.
Abyssal Mask and Void Staff for lowering enemy magic resistance, gettting more magic resistanse and dealing more magic damage to them and combined with Sorcerer's Shoes this combination will make tons of damage.
Use Deathfire Grasp for some massive damage on multiple enemies (handy in teamfights)
also add Rabadon's Deathcap for moar AP (recomended).
Get rylai`s crystal scepter to harass and slow your enemyes with Siphon of Destruction
Use mejai`s soulstealer if you havent got full build and you`re very fed.
Get Lich Bane for a lot of damage with your Mace of Spades
Add locket of iron solari for more magic resist and armor to you and your teamates (effective if allies have it but not recomended).
Buy zhonya`s hourglass (recomended) or Guardian Angel if they can kill you easly.
Aloso add Banner of Command for farming with minions and pushing lanes.
Add Randuin's Omen it`s very good for escapes.

So you need the deffencive ones for some deffencive stats.
You need Butcher , Destruction and Spellsword (the more Ap the better) for pushing and farming;
you need Blast , Mental Force and Archmage for some (not a lot you know...;)) AP.

you need the the health runes or the armor runes for initial help since Mordekaiser is ALWAYS FOCUSED and Very squishy at early-game.
Abilty Power runes and magic penetration runes for more damage and magic resist for more resistanse versus Ap emeyes.

Iron Man
35% of the damage dealt from abilities is converted into a temporary absorption shield. Shield generation is only half as effective against minions and monsters. Maximum shield strength is 90 + (30 × level). The shield decays by 3% per second.
Mace of Spades
Mordekaiser enhances his next attack, converting it to deal magic damage and replacing the base attack damage component with a base amount of damage. He swings his mace with such force that it echoes out, striking up to 3 additional nearby targets.
If the target is alone, the ability deals 65% more damage.
Creeping Death
Mordekaiser creates a protective cloud of metal shards to surround an ally unit or himself for 6 seconds, increasing the target's armor and magic resistance and damaging enemies within 250 range.
Siphon of Destruction
Mordekaiser afflicts all enemies in a cone in front of him, dealing magic damage.
Children of the Grave
Mordekaiser deals magic damage to a target enemy champion and curses it for 10 seconds, dealing additional magic damage over time and healing himself for the damage done.

If the target dies while under the effect of the curse, their soul is enslaved as a controllable ghost and will follow Mordekaiser for 30 seconds. The Ghost cannot use any active skills or items and deals magic damage with its basic attacks.
During this time, Mordekaiser gains 20% of the enslaved champion's total AP and AD. This bonus does not affect the ghost's stats.

Pros +mordekaiser is allmost the best farmer and pusher in the game +mordekaiser is allso one of the baest duelers in the game +s*** lot of damage +lots of sustain +very powerfull late game

Cons -bad early game -very slow (cant escape from ganks and teamfights) -no CC without items