If the enemy team is mostly if not all composed of magic damage dealers, you can swap either the Sunfire Aegis or the Demonic Embrace for either Gargoyle Stoneplate or Force of Nature, preferably keeping the Demonic Embrace, since it severely raises your AP.

If there isn't a single magic damage dealer on the enemy team and every single one of them is physical damage only, you should swap out the Spirit Visage for either Thornmaill, Randuin's Omen or Dead Man's Plate.
You can also choose to swap Demonic Embrace for any of those items if you still need more armor.

For ARAM and Support, Knight's Vow is also a really good option if there is one ally you definitely need to protect which the enemy team is giving a heavy focus to.
Which item to swap out will depend on if the enemy team is more Magical or Physical damage based, but you should try to keep the Spirit Visage and Rylai's on you if possible.

If you are not being able to fight too much and mostly working just as a tank and need to be even more tanky, you should swap out Rylai's and Demonic embrace for any of the situational tank items on the list depending on who are the enemy champions and what items they are building.

If you are snowballing a lot and the enemy team mostly has magical damage dealers, you can make Spirit Visage earlier, not make Sunfire Aegis and then build Horizon Focus as soon as you finish Rylai's Crystal Scepter.
If they are mostly physical damage dealers instead, you will keep the Sunfire Aegis and not build Spirit Visage instead, doing the same of building Rylai's Crystal Scepter and the Horizon Focus.
This is also heavily advised if the enemy team has a lot of champions who turn invisible, since you will reveal every enemy inside the area of your passive.

Lastly, if the enemies have a lot of healing, you will either build Morellonomicon or Thornmail.
Which item to swap out and which of these two you will pick is a case by case scenario depending on who the enemies are, what they are building and who are your allies.

As a support, Mordekaiser focuses on making sure his E hits and abusing his W to tank enemies damage and heal back up.
You can also use your E to remove engage based supports like Leona and Nautilus from near your ADC or use it backwards while retreating to force your enemies to disengage.

While Mordekaiser's ultimate is useful either to turn the normal bottom lane's 2v2 into two individual 1v1 fights, where you wish to remove the biggest issue for your ADC and leave them alone with the enemy that is easier for them to fight, while keeping the biggest threat in check and reducing their health as much as possible, preferably leaving them alive with barely any health so that your ADC may kill them as soon as your ultimate ends.

Your ultimate is also useful to nullify a gank from the jungler, by using your E backwards or sideways to stop the enemy ADC and support and ulting the jungler so that they can't get your ADC and using the 1v1 inside your ultimate just to move as close to your tower as possible.

If the enemies dive you under turret, you can also use your ultimate on the enemy ADC, removing the biggest threat to your own ADC and making it impossible for whoever is tanking the turret to retreat or the turret is gonna focus their ADC that is stuck in tower range inside the Death Realm.

You want to raise your level of Indestructible as fast as possible, to increase your barrier to health conversion.

Abuse W a lot in and out of combat as pure healing tapping W twice fast without really using the barrier if you can get away with it mid fight or out of combat, if your opponent is far away.
Unless your health is below 20 to 15%, to which start giving preference to the barrier, then you want to be more careful not to use it unless unless you are sure it is gonna be back up again and charged by next fight.

Since W's cooldown only start when you either heal or the barrier timer ends, by double tapping fast and not using the barrier, you actually reduce the time for the next use of W.

By the time W is maxed, you should already have Jak'Sho finished, which gives 20 ability haste + 8 from the runepage that translate to around 22% cdr, leaving Indestructible at around a 6.25 second cooldown.

Even if you heal immediately after W comes up while out of combat with nothing charged on it it's a 47 health return every 6,25 seconds. That is like having an extra 7,52 health regen per second.
At full charge, that is a 344 health return.

Since you can heal so much more, you can just stick to your enemies for a lot longer and do far more damage with your passive than you would with your Q if it was maxed out by this point between each W activation.
This means that even if you deal less damage with Q and E, you are actually dealing more total damage due to the passive in the same time window.

Those values might not seem so big at first glance, but, if you compare it to rank 1 W, with 36 healing and around 10 seconds of cooldown, when you stretch it over 5 minutes, that is over 1250 extra health healed.

If you were to use it every time it comes off of cooldown without charging the barrier, rank 1 W at level 9 with Jak'Sho will heal around 1000 health over 5 minutes, while rank 5 W at level 9 with Jak'Sho heals around 2250, so it is actually more than twice the healing.

I wasn't the one who named this build...
MY ENEMIES DID!