The Challenge is just an event and will be playable only a few days. But maybe it is really hard to get 12 Wins because the Pro Players also want him! The first entry is for free and after that, you have to pay 100 gems to join in again.
The next fact is really good! if you play this challenge you will be able to use all Clash Royale Cards. That means you also can use Cards which you haven't unlocked yet.
For each Win, you will get the same rewards as in the Grand Challenge. The difference is if you reach 12 Wins you get an extra Legendary Electro Wizard!
All your Cards Level will be set to Tournament Standards, even when your Cards level is lower.