Activate only while your opponent controls 2 or more Set cards in their Spell & Trap Card Zone. Select 1 of them. It cannot be activated this turn. This turn, inflict 800 damage to your opponent each time they activate a Set Spell/Trap Card.
It's not limited to one per turn, it can be used several times if you are playing a burn deck. You can pick the card back from the Graveyard to your hand, there are many ways to do so. Then you attack with any monster, if your opponent dares to activate the remaining cards... omg, there will be a lot of damage.
Next time check your facts before commenting a card, you piece of ignorant mule.