It's Law of the Jungle.

The assault on Scoia'tael guerilla ended well for the Monsters, but within their ranks the fights never truly stop. And there, only the strongest and most cunning will survive...

In the new Special Arena mode, you will be able to create your decks with Monsters cards only. The event will last till Wednesday, August 8th, 12 PM (noon) CEST.

Good luck and have fun!