The Worldgate is an underground fortress built by the Creator and the Destructor, two powerful entities who were created by the Primearch. It is the "home" and "prison" of Oraclos the Worldholder, who is found in the Heart of the World. Only those who wield the Godgigum Chalice with all five gems may enter it.
It is suggested that the Worldgate will be the final raid of the A Legend's Legacy: At World's Turn expansion.