I am not that good at this game, take everything I wrote with a grain of salt.

This is a build that I thought up myself and has success and fun with, send any feedback as this is the first guide I've ever created.

Start of Game

Mordekaiser is relatively strong against most matchups.

The main combo you should be using in lane is E-Q-AA. In extended trades/all-ins use abilities off cd, preferably right after you AA. You want to always try to only hit your laner with you Q as it does a lot more damage when the target is isolated.

I try to get a solo-kill before 6/first back. I often use use Ignite and Flash to get early kills. I will take [triumph] normally as it helps me survive dives. Do not start a fight if they have already have backed and you have not, or if you know their jungler is near.

First Back

If you can shove wave before backing as you do not have Teleport to prevent sieging.

Try to buy Leeching Leer first back.

If I die or am forced to back before I can afford Leeching Leer my priorities are Amplifying Tome > Boots > Ruby Crystal, this varies between matchups.

Level 6-End of Lane

Mordekaiser's R is very powerful for surviving/killing ganks and has moderate outplay potential. Normally your enemy jungler will be a level under you, ult them if they are not level 6 yet. You get bonus damage while in your ult.

Riftmaker is powerful. Do not fight your laner if they have their mythic before you.

If you are ahead try to freeze wave. Your laner will either not get any gold or will be forced to trade, which you will win. If you have Ignite and/or Realm of Death up, all in as you will most likely kill your laner.

If you are far enough ahead and have enough pressure (like Ignite) you could siege their tower while they are in still lane.

After you push your wave you can invade your enemy jungler if you have vision on their enemy midlaner and are confident you can beat their jungler in a fight.

Teamfights

As you normally wont have teleport will end up sticking with your team a lot. Try to ult their carries, at around 3-4 items you will be able to kill most squishes with a single combo and 1-2 AA.

Split Pushing

Only try to split push when you are ahead, you are squishy so you won't survive most ganks if you are behind.

With this build you will be able to one shot the caster minions with E-Q if you hit both, and by 5 items you will be able to kill casters with a single Q. Since you won't have teleport attempt to stay in lanes next to objectives so you will be able to assist in teamfights. Prioritize a tower over first two dragons, if you can't make it to a teamfight continue split pushing.

During ganks you have two options, disengage if possible, and fight. Target backline over anything else, ult and kill them if possible to make yourself stronger for the rest of the fight. During ganks that consist of only Bruisers/Tanks target the bruisers as this build has no MP. If you are fighting your laner hopfully y

Lane Trading/wave clear: E-Q-AA, if you have Lich Bane use E-AA-Q-AA to put it on CD faster.

Teamfights: E-Q-AA, If you don't have Ult disengage after your abilities go on CD, repeat. Use W whenever you get low or to survive burst.