Also, just to expand on Aerophoenix's post/ this whole Moonblast thing.
If you take a 0 EV/ 31 IV Salamence as your target;
Then you have a Dragon-type placeholder acting as Sylveon with 0 EVs/ 31 IV;
You can roughly get an idea of how powerful the attack is. I used Draco Meteor as Sylveon's attack, but reduced it's power to 120 (as we don't yet have any common moves with a higher base power than this without a negative drawback (which Moonblast doesn't appear to have)). From this, we can see that:
130 Base Special Attack Sylveon using 120 Base power Special Fairy STAB against 0/0 neutral Salamence: 105.74 - 125.07%
As you can see, it
only just guarantees the KO. If you reduce Sylveon's special attack to base 110, then you only have a 50% chance to KO; it does 92.44 - 108.76%. So, given that we know Salamence is OHKO'd by Sylveon's Moonblast, then (I think) Sylveon has to have at least 110 Special Attack, and if so, then Moonblast would have to be at least 120 Base power. If Sylveon has 130 Special attack, then Moonblast could be a bit weaker, but then it wouldn't ensure the KO.
(By the way; I'm making the assumption here that Sylveon will have the same stat numbers as the other eeveelutions. I also used this damage calc:
http://pokemonshowdown.com/damagecalc/ )
Apologies if stuff is wrong here. Maths isn't my strong point ;)
EDIT: Whoops, that Sylveon was with a - SA nature. My bad; the calcs are actually:
Sylveon @130SA: 117.82 - 139.57%
Sylveon @110SA: 102.11 - 120.24%
Sylveon @95SA: 90.63 - 106.94%
All using Moonblast as 120 Base power
Sylveon - 0 SA, 31 IVs
Salamence - 0/0 31 IVs