Hello, this is my first guide and it will be about mordekaiser. He has been my main champion for a long time and I want to share you my build of him and hopefully help you further out of low elo places because morde can carry as solo top very easy.

have fun reading.

The following runes I prefer using as you can see above:

marks: magic pen
seal: AP per lvl
glyph: AP per lvl
Quintensence: Flat AP

why these runes?

The magic penetration runes a pretty obvious, you need them without you wont be able to deal damage and get your shield up.
Seal Ap per lvl: I choose seal ap per lvl here because mordekaiser is a late game monster, with these runes I maximize the amount of ap I can have late game.
Glyph ap per lvl: same as above
Quintensence: I have mentioned above that mordekaiser is a monster late game, but with the quintensence I prefer having atleast 1 flap ap and I think the best is 3, just for the early game dmg.

As you can see above I play 9-0-21
I think as mordekaiser you really need the cooldown from the ultility tree, also the 15%cd reduce on ignite is very good for morde, because you can use the combo Ignite+ult more often this way. The 9 points in offensense are for the magic pen which you really need as morde doesnt benefit from a void staff or items like that.

Start of with a regrowth pendant and a health pot. this way you have the best survivability and it fits in the later game build also. When you're getting low or you are low or dead, depending on the gold get normal boots + ward + health pot and if you have the money hextech.
Hextech is needed by morde. you might be thinking why? well morde his ult has a 9% lifesteal and with hextech this pretty much can save your live when you're being ganked under tower. after the hextech get sorcercer's shoes.
Now your build has to be build on who is playing against you, if you have to play against a heavy AP team get force of nature but otherwise get rylai's first.
if you're playing against heavy AD get a thornmail first, then force of nature and after that rylai's.

After you have these 3 items you have 2 free options, I prefer taking ap/mr or ap/armor items just how the team is based. but you can also get a rabadon if you think you're survivability is high enough and you need damage.

Q: Mace of spades
W: Creeping Death
E: siphon of Destruction
R: Children of the Grave

As you can see above you need to max out siphon first, because this is your main damage and it also has range. Second most important skill is your Mace of spades, max this out second. and your third skill gives armor/mr resist this is pretty good because your armor/mr stacks also helps blocking damage of your shield ( passive )
Always take a point in ult when avaible.

When to use your ulti and on who?
You need to use you ulti on a ranged dps For example: Caithlin, ashe, corki, you know what I aim for. AD carrys, because these simply do the most damage.
You have to use your ultimate if you're sure you team will get the kill on the ad carry within 2seconds or you use your ult to get hp under tower when getting dived and hope you can survive by attacking them with all other skills. The ulti pretty much saved my life alot of time when I was being ganked with low HP under tower and I had kills for that also.

Ignite + flash

These 2 are the best 2 because when you see someone low under tower, flash in ult + ignite = kill

You can also play ignite+ghost but then you need to change one point in masteries to ghost. but I dont like this because you cant escape as good as you can with flash and you will have less chances to kill someone with low hp under tower.

With morde you can farm very easely, You see alot of mordes pushing. but this is something bad, try and last hit and make smart use of your E spell, try and get it hit only on one minion which needs a last hit. When you have your hextech I would advice pushing with a ward, because you're pretty strong at this point compared to others.
so, just like every other champion focus on last hits but make smart use of your skills to get them down with a last hit.

I hope you like my guide. Have fun playing mordekaiser and vote up if you like the build :D