DISCLAIMER: This build wasn't invented by me, I only wrote it down in this guide. All credit goes to HappyChimeNoise's video and Furbo for finding this build.

The best items for Mordekaiser are usually Riftmaker and Demonic Embrace. In the jungle, you have the Challenging Smite, so you won't need any more sustain. The reason behind Iceborn Gauntlet is the tankiness and slow on a low cooldown that makes you hard to kite. Iceborn Gauntlet is superior to Sunfire Aegis because it is 400g cheaper and it is easier to keep the enemies in the passive range.

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This item makes you very hard to duel due to true damage and reduced damage from the champion you smite.

These Boots give you a big amount of cooldown reduction, so your ghost and ult will be up more for ganks and fights. They are also cheaper than Sorcerer's Shoes.

After this item, you have survived the horrific early game a top Mordekaiser has, with the exception that you can just powerfarm to this item, which makes you hard to kill, and will make the enemies very slow combined with your Death's Grasp.

This item is so good on Mordekaiser, that I think it is trolling to skip this one. You should get it after your mythic item, as it gives you AP and HP, the complete max% burn and the HP to AP conversion. After this item, you can fight multiple people at once due to your Passive and Demonic Burn.

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This item really embraces the brainless mentality. With this item you get great on-hit damage and you can even miss all Obliterate, but still kill them. When buying this item, it is usually because you have too little damage in your team.

This item is great against AP Threats and will give you better movement speed against kiting.

This item is great against AD Threats and healing, although it should be brought purely for antiheal, because Oblivion Orb does a better job at applying the antiheal.

This item is perfect when ahead, as it will increase your damage exponentially. When you are behind though, this shouldn't be an item to build. If you need more damage, you can get a cheaper AP item like Morellonomicon or even Zhonya's Hourglass.

When the game starts, you want to full clear as Mordekaiser, as it is very healthy and easy. You want to start at red buff and full clear to the Rift Scuttler. This will give you around ~900 Gold, so ganking a lane and taxing 2 minions enables you to go back and grab Ionian Boots of Lucidity. If there are any free kills, fell free to take them with Ghost. After getting your Ionian Boots of Lucidity, you want to powerfarm until you have your Realm of Death, and gank preferably when your Realm of Death is up. When you used your Realm of Death, you should focus on farming or objective taking. After getting some kills and Iceborn Gauntlet, you can duel anybody with Challenging Smite + Realm of Death. This is the beauty of this pick, because even missing all Obliterate will still kill the target with Autoattacks and your Passive. You can even kill a fed enemy carry, but then you'll need to hit at least one ability.

Generally, you get countered by champions that can kite you or have high mobility. When ulting, always know if they have Quicksilver Sash or Zhonya's Hourglass, and check if they can hide somewhere. For example, Zeri could jump over a wall you can't reach, and your Realm of Death is wasted. Ulting an unsuspecting support will still give you the same results of stealing their stats, but you can be certain that you can kill them in the 7 second timer of your Realm of Death.

Apart from the kiteability, you scale better than most junglers and will be the carry. Ulting the enemy carry (If they can't counter it) will yield you with winning the fight 95% of the time, if you can kill them fast enough. At 6 Items, you are a raidboss that can't be dueled or kited when you have Ghost. To reach that late game, you want to get good cs and cood ganks off to assure you have enough gold for your items.