Hey everyone this is Eddy again, and this is how I achieved a 80% winrate in ranked as Mordekaiser.
You want to either solo mid or solo top with a jungler.
Doran's shield + health pot to start
Finish with Force of Nature / Boots / SF Cape / Abyssal Scepter / Rylai's Crystal Scepter / Comedy Sixth Option [2nd sunfire, randuin's, void staff, lichbane, whatever you'd like it don't matta]
If they're AP heavy - get FoN + Abyssal ASAP
If they're AD heavy - double sunfire bay-beyyy
If they're mixed - FoN + Sunfire ASAP, then your AP items
Skill priority: R-E-W-Q. Q got buffed recently but it's still not better than W or E. I've tested both leveling R-E-W-Q and R-E-Q-W and the former just feels a lot more solid, although the latter gives you a pretty underrated burst around the midgame.
Rune discussion: The marks are debatable between magic pen or magic resist; I go magic resist to deal with lane harass easier. The glyphs are also debatable between magic resist and cooldown reduction; I go cooldown reduction since my magic resist is usually pretty set with the marks and the Force of Nature.
Don't stack HP on him unless the HP is attached in some way to mitigation (basically, get Sunfire Capes, possibly a Randuin's). Against an all-AP team is when you might consider just getting a Warmog's, but not until you've gotten Force of Nature / Abyssal Scepter.
In my opinion Warmog's is a completely wasted slot on him. Thornmail is only good if the enemy team decides that you're the one worth focusing down over everyone else on your team, or if literally the entire enemy team is full of AD carries. SF Cape takes advantage of the fact that you don't have a taunt, doing damage to someone whether they're attacking you or not. In my experience, people try to just ignore Morde and go straight for the back lines. Thornmail won't do any good in that situation, whereas SF Cape will. SF Cape + Rylai also gives you all the HP you'll need. Warmog's gives you no mitigation, which means your shield is much softer. Having a massive HP pool as well as TMail honestly means that the enemy team is even more likely to ignore you.
For some reason people argue getting Frozen Mallet, because it applied to your ghost. Well, they fixed it so it doesn't even apply to your ghost's attacks anymore so it's worthless. Don't ever get it. Get Rylai's instead.
I find Rylai's to be immensely more useful than Mallet, simply because it helps getting INTO melee range. Not to mention all of his spells will proc Rylai's 15% aoe slow. Frozen Mallet is worthless if you can't get into melee range to begin with, and +200 hp is meaningless at the stage of the game where you're deciding between Rylai's and Mallet. Rylai's aoe slow is not as much as a single target FMallet slow, but getting two or more people with one spell cast is pretty useful for the team. Unless you're building attack speed morde (lol) Frozen Mallet is not nearly as efficient as Rylai's is in terms of both damage and the slow. Also, Attack Damage. Wut? You're getting much more bang for your buck with Rylai's nice amount of +AP than Mallet's tiny amount of +AD.
Let's look at the items:
Rylai's Crystal Scepter:
3105g total cost
Amp Tome + Blasting Wand + Giant's Belt
15% multi-target slow, 1.5 sec, which is the only effect you'll get as morde unless the person you Mace of Spades is alone
3250g total cost
Phage + Giant's Belt
30% slow on hit, 2.5 sec
With FoN + Boots, nothing's gonna outrun you with either item unless it's Singed or Udyr, in which case why are you even chasing
So the slow for both is pretty good. FMallet you can only apply to a single champ, and only as quickly as your (slow as hell) auto attack can hit. Melee Range. Rylai's every spell utilizes and applies the slow on, applying on multiple champs with Q, W, and E. It gives only a 15% slow for his ult, which is understandable seeing as how it's a perma-slow dot.
So we get to the other bonuses.
700hp vs 500 hp. With mitigation and health regen this is a non-issue. Your effective HP changes by like, 5%? depending on which you choose. Again, not really the issue.
So, we get to the real meat of the matter.
+80 AP vs. +20 AD.
+80 AP with 3 abilities that scale with AP: .2 Creeping Death, .6 Siphon of Destruction, and a tiny .02 base + the dot (1.7% + .002) for CotG.
+20 AD with 1 ability that scales with A 1.0 Mace of Spades
Keep in mind Mace of Spades also procs Rylai's 30% slow if the target is alone.
So what I'm trying to say is getting AD at all on Morde is a waste, regardless of what the recommended items tell you to get. Rylai's slow hits with every single one of morde's spells, and can affect multiple enemy champions. It also causes Creeping Death, Siphon, and CotG to all be more effective (and also be more effective recharging your shield). It also makes Mace of Spades proc the slow at the full 30% if the target is alone. The HP discrepancies are worthless considering you already have FoN + Armor items which are incredibly more efficient and miles better than stacking HP is. The AP and slow makes you more than a pest with low-mid damage; it makes you truly into a disruptive force. It is in almost every measurable way a better item to get on Morde than Frozen Mallet.
People say he can't initiate and therefore he is not a true tank. This is how you effing initiate with Morde: Pop ghost, throw Creeping Death on yourself, run in and Siphon + Mace the nearest squishy. Ult+Ignite if they get low. They *have* to blow spells on you or else their squishy is dead. The rest of your team rushes in while the enemy's ranks are in disarray. If you have another tank on your team you rush in alongside that tank and cause as much mayhem as possible.