Hello, welcome to my Mordekaiser guide!
This one is extremely short but there isn't a lot of technical information I think is too necessary especially since I'm not Challenger and yet have found a lot of success playing Mordekaiser this way.
Mordekaiser is a great, low difficulty, champion but is quite immobile and so might not fit certain playstyles but is a great pick to keep in your back pocket for his well-rounded potential.

*I will preface this by saying this is NOT high elo strategy!!*
It is just an insight into what I end up doing in most of my games and can be improved a lot.

Early- Play aggressively against champions you can out duel with Conqueror but be careful of enemy champs with Lethal tempo or Conqueror as well. If you have a hard matchup like Vayne, just stay back and farm safely (wait for gank or play slowly to catch them out when you're near your turret).

Mid- By now you should have a full item or two, tier 2 boots(if all goes well). Look for 1v1's and take enemy jungle monsters or ambush enemy jungler for a 1v1, if you've already taken the top turret. If you have not taken the turret yet, push your lane and chunk it down. Help secure objectives and picks.

Late- You're likely a dueling nightmare by now, split push if need be but definitely help team secure barons and drakes by ulting enemy jungler to prevent smite steals for example.

I won't bore you with too much information here.

Mordekaiser does really well with high damage items like Rylai's Crystal Scepter, Riftmaker, Liandry's Torment and I would say these items are his bread and butter core.
Other damage items like Hextech Rocketbelt (magic pen) and Zhonya's Hourglass (survivability) are more situational and can totally be build alongside the core items.

Spirit Visage: Personally I end up building it in 99% of my games just because it feels great with Indestructible. A favourite on Mordekaiser.

Sunfire Aegis: Great synergy and good with Grasp of the Undying

Thornmail: Against heavy healers

Randuin's Omen: Against high crit enemy

Jak'Sho, The Protean: A well-rounded defensive item

Force of Nature and Kaenic Rookern: Against heavy AP

Abyssal Mask: Great against heavy AP whilst also giving you a damage edge with it's passive

I am not a high elo Mordekaiser player, this guide is just a compilation of what has worked for me :)
The laning advice can 100% be expanded on and made better, feel free to leave your own tips if you wish!

Thank you so much for checking out the guide and good luck out there!