Yes, it is like Un4Givens, but ive never been one to stack sunfires, it just doesnt suit mord well. This setup keeps you pounding your opponent, period. With Randuin's on top of the CDR runes, your CD is reallly low. You can prettymuch mace and siphon everyone to death, and keep your creeping death up constantly.

Thanks for the input.

The item build is basically like Un4Givens, i see you only tried to switch the second sunfire for a Randuin's since the sunfire got nerfed.
But you changed the runes, and I think the cooldown reduction can be handy, since ive died many times and if I had stand like 0.5 sec more and attacked once again I would have survived, so I guess the runes fits him well.
I think since you got no defensive runes you should mayb switch your boots for Ninja Tabi or Mercury, depends on the other team, afterall, Mordekaiser is a tank.
Nevertheless, good build +1