Mysterious Portal is an event that occurs randomly, which will create a small graveyard with a red portal inside on an Island. This new portal requires Green Portal Stones to activate. You can go through this new portal, and this will take you to a hidden Island. Unlike the seasonal levels this event seems to occur randomly, not on a date we can set our system to.
During the Mysterious Portal event, the Mummy and Ancient Mummy enemies around the new portal graveyard may drop the recipes for the following: Blue Carpet Block(Ancient Mummy), Green Carpet Block, Gold Carpet Block, Orange Concrete Block, Green Concrete Block.
During the event, two Blue Lanterns will spawn on the fence surrounding the portal. It's also possible for several Blue Lantern to spawn in the structures on the Island beyond the Portal.