It's a very rare mask called Luchador, and it can only be obtained through the Twin Gods gifting it to you. There are four versions - blue, orange, green and red. I think you may have seen Flyleaf with the blue mask, it's quite distinctive. Hope this helps!

The Twin Gods usually appear on certain holidays, when they change the Forest and sometimes hold little 'parties'! It's a rare chance, but if you find the Twins on those special days, you might have a spell cast on you.