Riot, just promise me ONE thing; When you finally start working on Morde's VGU...
Make your designers listen to Metal 24/7 Don't let any thought about Morde not be accompanied by heavy metal shredding and lyrics of unfathomable vile powers. Glue headphones to all their heads if you have to.
Trust me, only good things can come from this.
Real talk though;
Mordekaiser's "Master of Metal" theme, as much as it's simply too goofy as a champion's only character trait does resonate well with the champion he's supposed to be. Metal itself has a lot of themes that just go hand in hand with dark Overlord characters like Mordekaiser.
Heavy riffs and powerful guitars that bring out the might and unholy power a being like Mordekaiser represents. Dark Lyrics of beings beyond human comprehension, of vile wraiths, wicked powers whose malicious intent and unholy goals have been the topic of so many Metal songs.
Just take a look at classic 80's tracks. Every time I just kick back and listen to things like Metallica, Dio, Judas Priest or Megadeth I get the feeling most of their tracks could be used to describe Mordekaiser in a certain way. Be it the vile creature feeding on torment and suffering as shown in "Harvester of Sorrow" The undying revenant beyond human comprehension like Black Sabbath's "Iron Man" Dio's ballad "Metal will never Die" that presents metal as just another unholy power that will never cease to exist (just like Morde himself)
Fun talk though;
You better not disappoint in what obscure metal references you may be including in not only his normal VO but especially in his Pentakill voice. Hell, you gave Karthus 7 minutes additional lines and made him metal as heck, better amp it up and make Morde's frigging eleven times more metal.
Bonus points if you give him Champion related taunts that reference Metal songs.
to 
"You're just a rainbow in the dark"
to 
"But your castles crumbled and you're left with just a name; Where's your Crown, King Nothing?"
to 
"You're just a crazy train"
to 
"Punk tried to kill the Metal, but they failed as they were stricken down to the ground!"
killing someone with his ultimate (if he still retains some soul enslaving aspect)
"Master of Puppets, I'm pulling your strings"
And maaaaaany more.
Just wanted to get this off my chest^^
(or any other champ)
All of us get lost in the darkness, dreamers learn to steer by the stars. (rush - the pass)
The first blood I spilled was the blood of a bard I had to wipe the smile away (amon amarth - first kill)
We were never alive and we won't be born again (slipknot - dead memories)
let me tell you something baby, you love me for everything you hate me for (in this moment - whore)
If you like to gamble, I tell you I'm your man, you win some, lose some, it's all the same to me (mothorhead - ace of spades)
(ok) The earth starts to rumble World powers fall (megadeath - symphony of destruction)
Against the world in which they have to live and all the hate that's in their hearts (white zombie - children of the graves)
METAL IS PERFECTION.
/
and/or 
"Holy Diver, you've been down too long in the midnight sea!"
Sweet dreams are made of these. (Manson ver.) @ 
Exit light. Enter night. etc etc
