Offense is your defense. Keep fighting to build up larger Indestructible shields.
Hitting multiple champions with the same ability can help quickly activate Darkness Rise.
Use Realm of Death on a low health enemy to guarantee the kill and keep their stats for the rest of a teamfight.
Mordekaiser builds up a powerful damage aura when fighting champions, so keep your distance.
Damage he deals can be converted into a large shield, and consumed for health
Realm of Death will cut you off from your teammates completely. Try to save mobility spells to escape Mordekaiser once inside