Silver Dust Storm [single]

[Lonk]

Silver Dust Storm

Normal Spell Card

 

Discard up to 5 cards from your hand. Select and destroy Spell and Trap Cards your opponent controls equal to or less than the number of cards you discarded.

 

Discarding cards isn't a cost for this card in case if you ask.

[-Griffin]

-1 Heavy? :s At least. Worse if you have any targets. I can only see this as even slightly usable in Fableds, and probably not in there.

[Skarlet]

Dark Worlds. Next.

[Manjoume Thunder]

Your opponent then selects and destroys Spell and Trap Cards you control equal to or less than the number of cards you discarded.

 

This part makes it bad. Remove it and the card will be playable.

[Lonk]

Since everyone says it (or is at least thinking it), I am removing the part where your opponent selects the cards.

[Zowayix]

You might want to make it destroy S/T cards equal to one more than the number you discarded. That would make it about as strong to Heavy Storm as Lightning Vortex is to Dark Hole.

Also, to make it clear that discarding isn't a cost, you should change the beginning to:
"Discard up to 5 cards from your [b]hand, then[/b] select and destroy S/T cards..."