We're all looking at this the wrong way.
Master of Metal and Lord of Death are both stupid names.
Mordekaiser is clearly the King of Pop.
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Seems more like he's into "Death Metal" to me.
Meanwhile, at the Legion of Doom Riot Headquarters:
CertainlyT: I got the VU for 
finished >:p
http://media.giphy.com/media/frgHDDBbLhEzu/giphy.gif
Riot big wig: Perfect! once this goes through, the process of removing all traces of metal from mordes Identity will be complete and no one will play him again! >:D http://cdn.rsvlts.com/wp-content/uploads/2014/06/maniacal_laugh_austin_powers-compressor.gif
...I will be honest.
I would favor King of (Heavy/Death) Metal over Master of Metal.