Mercurius Gate

The Mercurius Gate is a giant door behind which an unknown inhabitant of the Cathedral City sealed away the black entity, or Black Flower, due to its threat to the world. A small portion of this entity escaped being sealed and later became the flower that infects Zero's body.

The Gate has two magically protected doors sealing the inner room, making the Mercurius Gate the 3rd and final door holding the entity back. It is unknown what else may lie beyond the gate, but the Lords of the Land intend to open it in order to obtain great power.