Well morde is a juggernaut. Sort of intended to run tank items.
I’ll get back out of bed and go build a rume page for you.
Now keep in mind two things - mordekaiser is one of the weakest juggernauts, and largely a flop.
Secondly, runes are very playstyle oriented.
My first suggestion is pretty basic for juggernauts.
Always start ruby crystal or dorans shield.
First of all, your secondary rune should ALWAYS be resolve.
Pick one:
Resolve: Iron skin, Second wind.
Resolve: Unflinching, Second wind.
Secondly, the primary runes:
Sorcery: Summon aery, Nullifying orb, Absolute focus, Scorch.
Precision: Fleet footwork, Overheal, Legend:Tenacity, Coup de grace.
Domination: Electrocute, Taste of blood, Eyeball collection, Ravenous hunter.
Inspiration: Glacial augment, perfect timing, magical footwear, approach velocity.
Sorcery: Phase rush, nullifying orb, celerity, waterwalking.
Feel free to ask if you need more help.