The Monstrous Chest is a limited edition Chance Chest that was released on October 10th, 2014 and ended on October 13th, 2014. According to the banner, the Wingstorm Battlegear was released in this chest; but however this was proven false since the Wingstorm Battlegear was available in the Flourishing Chest.
1 Chest: 35 Gems
11 Chests: 350 Gems
40 Chests: 1,200 Gems
11 Chests: 5 Air Fusion Boost Armors