The idea here is more or less an Off-Tank DPS. This build is 1) meant for Normal Games and 2) not meant to be the only tank.

Haunting Guise is severely underrated. It is a very cheap item that has everything Morde wants (maybe not needs, but wants..). Health, AP, Mpen. Nice. This frees up rune slots for whatever you want - you don't need MPen with Guise and Sorc Boots.
The idea behind Leviathan is also that it is cheap, and gives you the extra bit of health you need - this idea behind this build is to have survivability from Sunfire or FoN and Leviathan while building up MPen and AP.

Why all of the cheap items? The aim is a strong, sturdy early-mid game. Instead of stacking expensive tanky items, get a sunfire, grab some MPen and health, and get some kills. You can easily afford all the way up to Abyssal Scepter by 35 minutes.

Besides, with 20 stacks on Leviathan, you can't be killed. It just won't happen.
And even if you can't get 20 stacks, if you can maintain a measly 8 stacks, that's 436 Health, for 1275 gold. Just over Giant's Belt.
You're right, with propper shield generation, you won't need crazy amounts of health, which is why I don't have a warmogs up there - OR even Rylai's, which is an expensive item for little gain. Giant's Belt is 1/3 the cost of Rylai's and has 430 Health. Rylai's costs an extra 2k gold for 80 ability power and 70 health. I'd rather build Abyssal Scepter with my Blasting wand.

As for the Gunblade, I grabbed it for the AP and vamp, mostly. It was my last item, the game had gone on for quite a long time, and I wanted that extra bit of survivability while tanking 3-5 fully fed enemies. Besides, the slow on it is more than Rylai's, less restricting, and does damage. Honestly, anything would have worked in it's place; Lich Bane, Deathcap, FoN, GA, you name it. I was building on a whim.

Sorry for the wall of text. In conclusion, I have a proposition for you. 1) Either try this build in a game, and revote/recomment, or 2) actually read the whole guide, before making a decision based only on my skill order and items. Please, if you don't mind. This was my first guide here, and I don't want downvotes simply because it LOOKS different or SOUNDS like it might not work.
Thanks.

I'm not sure if you're going for straight DPS mord, because if you are, this isn't terrible. But, if you're trying to be tanky, you don't have the best items for that spec, utility-wise.
As a tanky character, you shouldn't have a set build in mind.
For instance, I like your opening, but i don't see a point in either Haunting Guise or Leviathan. You have enough MP without the guise, and you don't need a crazy amount of health, negating the need of Leviathan. A better choice would be something to allow for a little bit more damage, while providing health(I really like rylai's for this). This extra MD feeds Iron Man faster(negating the need for 3k+ health) and makes you bulky too.
Also, I really don't see a point in investing any gold in anything that gives Mord AD. He attacks so slow that it's almost not worthwhile. (Considering the Gunblade in your screenshot)

Any other questions? --- Sorry for the curt comment from before. I'm pretty sure I was up for like 3 days trying to get tired so i could sleep.

Really? Why? Because it's actually different than most Mordes? Is that against the rules? You say it is wrong, but offer no reasons why, or any proof, or explain how to fix it.
I honestly don't understand your reason to downvote. Please explain.

You've got the skilling order down, but your item build is all wrong.

Thank you very much! I was afraid people would shoot it down without thinking about why it could work, or without trying it.
Also, looked over your builds. That Cait build looks really solid, I like it. Will definitely give it a shot and drop you a comment!

Dude great build Love your ideas on Mord.