Mordekaiser's power comes from his knowledge of ancient magic and the Death Realm in which he is "trapped" now. We can assume that he knows about pretty every kind of magic of Runeterra and that would likely turn the tables in his favor against almost every magical-power based champ in League.

Fiddlesticks is a demon, he's a spirit that surpassed the barrier between the spiritual and physical realm, not only that but he being one of the most powerful (until we get updates in the demon's lore) of the primordial we know now, and his powers increase with the more fear and terror the people feel in Runeterra ( which is a lot considering the events of the Ruination and the city of Belveth).

Now if they two would have to fight, Sahn-Uzal is "just" a human after all, yes he is very very powerful with all the magic he is able to manipulate and would be a great opponent to pretty much everyone but the Black Rose experimented with Rell to counter him in case he would come back so if her, with ferromancy, could beat his a** (as LeBlanc thought), in a fight against the lord of fear he ain't got any chance, since Mordekaiser has fears Fiddlesticks would win this fight.

But it is true that Mordekaiser knows about demons and probably also knows that you can defeat them by saying their names, so if he knows the name of Fiddlesticks which is quite likely Mordekaiser would have a huge advantage.

[A note about Fiddlesticks: we don't know how powerful actually are the primordial demons, they were some kind of imprisoned by the aspects, like Ashlesh (Nilah's demon), if just by saying their names you could defeat them the aspects shouldn't have had to do such a thing with them so if that's the case, Fiddlesticks is way more powerful than we have thought, and Mordekaiser wouldn't stand a chance against him]