Deals damage. Power is equal to 20+(X*20), where X is the user's total stat stage changes that are greater than 0 (including evasion and accuracy), for a combined maximum of 860. Does not decrease in power if any stat drops below the level +0.
Stored Power starts at 20BP, and every Calm Mind will add 40BP. Moonblast has a flat 95BP, boosted to 142.5 since it's STAB.
20 60 100 140 180
You'd need at least four Calm Minds for Stored Power to deal more damage than Moonblast. Moonblast is almost always going to be the best option unless you're running a Baton Pass chain to give Sylveon extra boosts to bolster Stored Power.