Hey guise!~
I've just experimented with diff variations of building Morde and wanna share it with u.
It's my 1st guide, so don't judge too hard :)

Let's make a look for runes. I took the most popular runes for AP champions, so it paid attention mostly against AD top laners, like Riven, Irelia, Jax, that all do damage from AD. That's why I didn't take any magic resist.
Of course, we are going to spam a lot @ lane phase, so we need penetration and AP as much as well. I think, my AP runes fit to my play style, so if you wish leave 'as is' (scaling AP quints), go ahead. Or u can choose usual Quints of Ability Power. Which one is better for you - your choice!! In my opinion, Quints of Scaling AP make a little, but important meaning in late game. Just compare: 3 x Quints of AP - 14.85 AP @ 18 lvl, or 3 x Quints of Scaling AP - 23.22 AP @ 18 lvl.

Time for masteries! I will try to explain for each bridgestone and keystone, why and etc.

Set up for masteries - 18-0-12.

Ferocity
Bridgestone 1 - Sorcery. We mostly do damage of our abilities, so I prefer to increase damage of my spells, keeping out of distance our enemies and spamming with E as much as we can. If you want to make a fighter, who can stand long time smashing with mace and spells at the same time, you can balance with 1st Bridgestone. All in your minds and hands!

Keystone 1 - Double Edged Sword . Here we are not farmers, so Feast will be useless for us and we won't be able to deal damage as proper. By the way, if we read word by word, we can notice, that balance of dealing and taking DMG is 3%:1,5% for melee champions. This is perfectly fit for us.

Bridgestone 2 - Vampirism . Because of Mordekaiser's Q and E. They are scaling from AD and AP both, so we can totally have 4$ of vampirism as well, but don't take this number literally. This bridgestone will help us @ lane phase, mid game and late too, because we will rush sustain.

Keystone 2 - Oppressor . Should we wait for additional damage, when have to kill enemy faster? Nope! After getting Hextech Gunblade or Bilgewater Cutlass, we have a bit of slowing enemies, and it will be a point of using this keystone.

Bridgestone 3 - Piercing Thoughts . Penetration... Penetration!! PENETRATION, CARL!! Double, triple, no matter! If u noticed, we are not taking any item for penetration, so we are taking this stuff.

And last one, Keystone 3 - Warlord's Bloodlust . Why not AoE damage? Because we are building offensive tank, and our Q will allow to deal critical damage with every 3rd strike, which also will heal us within spell vamp and lifesteal. And this will allow us to stand in duels and in team fights longer! Thanks a lot to SparkleThunder
for giving me an advice, so we will pay attention then to dmg for 'removing' our enemies so faster as we can and we take Deathfire Touch !


Resolve
Bridgestone 1 - Unyielding . +5% Armor and Magic Resist. Getting fatter :) Ewww... Sounds awful, but we need it! 'Thin off-tank' - isn't ridiculous?

Keystone 1 - Tough Skin . Are we running from somebody? Nope. We are not in maze, we are on battle field!!! We are the warriors!!! >:O

Bridgestone 2 - Runic Armor . +8% for all healings, including lifesteal... Yummy!! :3
Also Spirit Visage will increase our lifestealing and spell vamp! Deal with it!!

Keystone 2 - Perseverance. Why this? Because I want we can have a good regeneration in case of waiting our jungler under turret! Profit? I think 'yes'!

That's all, folks! I mean with this chapter.

Next is items.

Okay, guys!
As you can see in sheet, I take Amplifying Tome and 1 x Healing Potion. Earlier, Riot recommended Doran's Shield. Why.... Why.... -_-
From this Tome we gonna rush for Hextech Revolver, which will give us sustain from abilities.

After this I usually take Vampiric Scepter, in summary we have 12% Lifesteal and 12% Spell Vamp. Isn't good? I think it's good! So we can stay on lane until getting kills or farming good, but getting ALL - best way to get fatter, stronger and scary!! >:O

After getting Hextech Gunblade, we are building Mercury's Treads.
'But there's no mage!' - say you. And? These boots give us Tenacity, which is good for all tanks. And additional magic resist will be also good for us. Cuz in enemy team should be at least one mage.

And now I wanna make an accent. If you feel, that you have advantage against your Top Laner, rush Spirit Visage, even he's AD, because Spirit Visage will increase our all healings. But if you feel often tones of pressure, rush Thornmail. They will kill themselves faster, and you will stay alive longer.

After building our 'fat', we building our damage. Here's Rabadon's time. I think this item doesn't need any description!!

And preparing for our last battles, we are getting fatter and stronger with Rylai's Crystal Scepter, which gives us not only HP and AP, but also effect of slowing enemies. Also it gives a synergy with our 2nd Keystone in Ferocity, we will give more and more damage! That's what we need :)

I hope you will try out this build and if you liked it and this guide, thumbs up, comment below and thank you for wasting your time for reading my guide!

Why there's no explaining of combats and farming? I think most of you, saw, knew or know how to play him, but may be you searched for something new for this juggernaut!
That's why I wrote this.

Thank you for attention, guys!!

And may the Force be with you!!!

P.S. If this guide isn't new for you, plz, no rage and such comments. I understand all, but there a lot of guides, which are not refreshed for this season.