I am not quite sure where I have obtained this information and did not believe it myself first but I think I've read it on two different sites as well as been told by a friend that such is the case with the Q's of both Syndra and Karthus.
Karthus I can understand if it's true since his Q acts differently depending on if it hits one or multiple targets but since Syndras' Q has no such interaction I found it curious, I will attempt to find where I've read it and link it.

Edit:
Oh yeah here we go, firstly I saw someone write it as a comment on Syndra on leagueoflegends.wikia.com which I thought nothing of since he had no proof of it, but after a friend of mine told me it's true I started thinking and then when I read the post below I figured "Hey, I guess it's true."

http://www.reignofgaming.net/blogs/a...l-is-limitless

Though he has edited the part about her Q by crossing it over.

Also I went into a custom game to check it out and, on minions at least, Muramana does not give any bonus damage to her Q.
Apologies for providing incorrect information.

So I assume that it did in fact to apply it earlier due to a bug which later got fixed.

Unless you spear people to the face, it scales so much better with ap, so why would you do that?

@OP:
No it is not. You get huge numbers on the ult (if you can pull it off that is), but outside of that it's pretty bad, because only ult and auto attacks proc Muramana.

Correct, her Q counts as multi-targeted not single-targeted no matter how many targets you hit with it.

He must be thinking of Karthus, when his AoE Lay Waste hits one target it procs it as single target (because of how it works with the bonus damage)

The math says Muramana is much superior to Seraph's on Syndra.

I've seen the numbers crunched by 2, maybe 3 different people, and the general consensus was that Muramana active was the equivalent of ~500 AP with no other Mana items.

Honestly though she doesn't need Tear to be mana efficient, Chalice/Athene's usually does the work just fine, even if she's sans Blue.

DFG, Muramana (its magic damage is amplified by DFG), Deathcap, Athene's, Void Staff, Sorc Shoes is a very, very scary build on Syndra.

If she can hit you with a 7 orb, DFG'd ulti, there's not much in the game that can avoid being one-shot. And because of the way her ulti works, not even spellshield can really stop it.

It's a pretty weird item. As only the active toggle on the muramana is what you want to use.

However, with the right build, although costly, her full combo is just something you cannot ignore.

I've once had the pleasure of this full build and a 6 orb combo:
Sorc boots, void staff, seraph, murmana, rod of ages, deathcap

I can't recall full numbers, but i remember a Vi crashign into my team, and she built tanky. She landed her ult. even with 3500 hp, and over 100 mres, it was the last thing she did. She went up, but never came down alive.

The damage muramana adds brings her ult to veigar levels of damage, but between building the tear and having the muramana, she's going to be behind in damage, instead of buidling pure ap

Except that you just proposed one of the least defensive builds in the game. Meanwhile, assassins are all over the place. I get that you might build slightly differently for this, but if there's an opposing Katarina you're asking to be a reset.

Yes, the Muramana active gives the equivalent of ~480-600 AP, but that is only on your ultimate. Plus, you really want to spam your moves in team fights so the odds that you are at full mana is pretty unlikely and therefore your damage output is less than 480-600 magic damage. Syndra's positioning late game is pretty tricky and if you just run up and use your ult on one of their carries you will probably kill them, but most likely you will die in the process. Her damage output late game is far less reliant on your ultimate and more reliant on her other AOE moves.

I promise you, a build of Sorc Shoes, DFG, Serpaphs, Deathcap, and then Void or Hourglass if you can get them will still kill people with a 5/6 ball, but you will be overall more helpful to your team when compared to Muramana.

I'll crunch the math later tonight since I haven't seen anyone post anything convincingly conclusive yet.

I'd assume that Karthus's skittles are multi-target, regardless if they hit one or more things.