
Reviewed for patch 7.3b

Favorable matchup
Reviewed for patch 7.3b
Fiora punishes Mordekaiser's lack of dashes: Riposte answers Death's Grasp and Lunge hits a Vital from an angle his mace doesn't cover. She wins the short trade, not the one that drags on inside his aura.
The zone pull of Death's Grasp is how Mordekaiser drags you into his striking zone. Riposte parries damage and crowd control for a moment, and Riot's description says the counter-stab stuns if what it parried was an immobilizing effect. Use it when you see the pull coming, not against a basic attack.
Darkness Rise only lights up its damage aura after he lands three attacks or spells. A Lunge onto a Vital, plus the healing and speed the Vital itself gives, lets you end the trade before Mordekaiser reaches that third stack.
Realm of Death isolates you both for seven seconds and hands him part of your stats. Enter that duel with Riposte available: Grand Challenge reveals all four Vitals and, if you complete them, heals you for a few seconds. With no allies inside, that heal is yours alone.
Trade when Mordekaiser misses Death's Grasp or spends it on the wave: without the pull he has to walk to you. If he still has pull and shield, keep the trade to one Vital and leave.
If you play Mordekaiser, fake the pull and wait for Fiora to burn Riposte before throwing it for real. Obliterate hits harder on a single enemy, so catch her away from her wave. Hold Realm of Death until Riposte is down: without the parry, the isolated duel leans your way.

Favorable matchup
Reviewed for patch 7.3b
Mordekaiser has no dashes: to hit you he has to walk up or land Death's Grasp. Kennen punishes him from range with Thundering Shuriken and leaves his zone with the speed of Lightning Rush.
Mordekaiser's kit swings the mace around him and has no leaps or charges. Thundering Shuriken damages the first enemy hit, so you can tag him when he steps up to last-hit and get back behind your minions before you are inside his pull.
Mark of the Storm stuns on the third hit. The shuriken, Electrical Surge and passing through him with Lightning Rush all add marks: one stun buys time to hit and get out before Darkness Rise lights up his aura.
Realm of Death isolates you with him, but it doesn't take away room to move. Lightning Rush gives a lot of speed in its first moments and Slicing Maelstrom adds armor and magic resist while it strikes: run, mark again and outlast the seven seconds.
Poke every time Mordekaiser walks to a minion and step back before he is in pull range. If he spent Death's Grasp on the wave, that's the moment to complete the three marks.
Mordekaiser doesn't win by chasing: wait by the wave, let Kennen spend energy and throw Death's Grasp when he comes at you with Lightning Rush. Indestructible banks part of the damage you deal and take as a shield; pop it just before the stun. Realm of Death keeps his team from saving him, so use it while you still have most of your health.

Execution matters
Reviewed for patch 7.3b
While Hallowed Mist is up, Gwen can only be targeted by enemies who step into it. Mordekaiser has to walk in before casting Realm of Death on her. It is a timing answer, not immunity: you have to read when he'll use his ultimate.
Realm of Death is cast on an enemy champion. Riot's description says Gwen can only be targeted by enemies inside Hallowed Mist; with the Mist up, Mordekaiser must enter before he can pick her, and that step gives you time to cut the trade or leave with Skip 'n Slash.
Thousand Cuts adds damage based on the target's health and heals you for part of it, and the center of Snip Snip! deals true damage. Against a fighter who stacks a shield with Indestructible, that damage isn't reduced by resistances, although the shield still absorbs it.
Fight when you have Hallowed Mist and Mordekaiser still holds Realm of Death: the Mist exists to deny that moment. If you already used it, avoid long trades until it comes back.
Against Gwen, order matters more than damage: cast Death's Grasp or wait for her to spend Hallowed Mist before going for Realm of Death. If the Mist is still up, walk into it so you can target her. Obliterate punishes a single target; catch her when she has no snip stacks left.