Not Updated For Current Season
This is the Build that i lean towards for Mordekaiser. I've played him a lot and from my experiences people generally don't have enough time to think when you're running around at 450 movement speed.
Ignite - This in combo with you're Children of the Grave is a deadly combination and you can usually finish off people easily with one hit of your Siphon of Destruction and popping these two on them if they're at about 45% hp
Ghost - Excellent for running your enemies down or escaping.
9x Greater Mark of Magic Penetration 8.55 Magic Pen from Runes, all of your abilities use magic damage in some way.
9x Greater Seal of Evasion - 6.75% base dodge from runes, helps you avoid all melee attacks, could even save your life
9x Greater Glyph of Scaling Magic Resist - 24.3 Magic resist@lvl 18. The MR is gradual but it helps throughout the game especially against casters like Ryze and Morgana who can keep you locked down temporarily.
3x Greater Quintessence of Vitality -- 145.8 bonus hp from runes, Helps a TON early game, espcially solo laning versus two. Also helps for those early game ganks when you have both your Children of the Grave , Ignite and Ghost up. It's almost a surefire kill if they're below 50% hp if you tap them with your Siphon of Destruction once :D .
I always start with the Regrowth Pendant and a Health Potion . In the long run it's better than any health potions you could buy.
First buy --> 1,100G-1800ish buy your Sorcerer's Shoes outright, if you don't have a hp pot on you i will usually grab another one because they can come in handy. If you have 1800g for your first buy buy a Chain Vest to get you started on the Sunfire Cape .
Second buy ---> 1810 - 2510ish. Buy your Sunfire Cape . Welcome to Mordekaiser Master of Farming. Farm as many neutral mobs as you can while watching your designated tower to prevent the person(s) you're laning against to get to your turret without you there.
Third buy ---> This one really depends on who you're fighting. If you're up against a heavier ad/as team pick up a Thornmail, It's not listed in the buy as i really don't use it unless i absolutely have to. If you're up against any amount of casters, this is around the time that i will say get a Force of Nature as your MR item because it also get you on track for movement speed. Start with the Negatron Cloak first.
After that, I typically pick up a Sheen Just for the sole fact that it makes your Q hit like a truck.
Trinity Force , This is an item of debate amongst Mordekaisers I've found personally I am a huge fan of this item for Mordekaiser as it will make you do redonkulous amounts of damage. the sheen will proc over and over with your abilities because they're on such a short cool-down. plus, no one really likes getting hit with Q when it deals a fat chunk of damage. Sheen > Zeal/Phage > Phage/Zeal (depends on if people are starting to run from you yet)
After you have your Trinity Force you should be running at about 450+ Movespeed and running down enemy champions with ease.
After you have acquired your Trinity Force . I usually get a Hextech Revolver . Reason being, it gives you a little bit of extra AP/AD and it gives you 20% life steal AND 20% spell vamp which as Mordekaiser makes your spells free. On top of healing you for about 20% of the damage you deal. It's pretty nice. I usually pick up the Hextech Revolver First and then a Vampiric Scepter shortly after just so that my Q will heal me for good amounts as well as getting heals from my Creeping Death and Siphon of Destruction , oh and yeah it affects your ult as well, so more healing from Children of the Grave .
At this time in the build I was very partial to an Abyssal Scepter but times have changed, At this point in the game the other team should be focusing you down as quickly as possible, it sucks I know. I usually pick up a Guardian Angel just to get the advantage in a team fight and push your team in their base for the win.
In summary, This build has been good to me and I've decided to share it. PLEASE leave a comment on this, I'm dying to hear how people are doing with this build. I love constructive criticism. Hopefully before rating, you have actually seen the pain train in action. :]