So, Luden's is absolutely hilarious on Mordekaiser.

I had said when the item was announced that Morde would be a prime candidate for it, but god damn.

So, reasons it's hilariously good on him:

1. He has fairly short CD, spam-happy, AoE spells.
2. Your W can proc it from any distance on any target it's on. Slap it on a minion and it'll proc it all the way across lane.
3. Movement speed is a godsend on his slow ass.
4. The ticks of your ultimate will proc it. This means that Morde can slap his ult on you, and for insurance, run around in circles spamming his spells cooldown to rack up charges and add extra umph to the ultimate as you feebly try to get away.
5. It procs spell vamp and Rylai's, both of which Morde regularly likes to buy anyways.

Also, anything that applies spell effects procs Luden's; the active from Gunblade (which I prefer over WotA for the active and noticeable Q damage) will trigger Luden's too.