The Opening of a Door

An Opening is a terrible event that unfolds after enough time has passed during which a Door was left to gather enough mana to be able to open on itself. This time limit is different for each rank of Doors, but all can open up nonetheless. What happens when an Opening occurs is a fluctuation in the mana within the area around the Door and then the it opens much like a normal door you would find in a home, but behind these are much more terrible creatures than a person. Once it happens, an Opening allows the monsters behind the Door to enter and leave the space freely.
The recorded openings have shown that when this type of event happens the monsters within begin to attack the nearest things to them, in most cases people are the main casualty when an Opening occurs. During this event the military and police are sent to evacuate the area and provide blockades and support until Divers are mobilized to deal with the threats coming out of the Door. The Divers that are contacted are known to be the Guild found nearest to the location but if others are in the area, the public nor the Association would disagree to assistance. This is especially welcomed especially when the boss of the Door decides to leave the space, though it is not common when it does happen those who have lived defeated the creature in a pile of rubble that was once a popular mall.

Manifestation

In the few minutes it takes for the Opening to complete, the magical pressure in the area increases and begins to fill its surroundings. When this happens the temperature, immediate weather and sometimes even the land changes if the amount of mana is enough to force a change.

Localization

The range of the changes around the Door is dependent on the rank of the Door and the saturation of mana that has built up over time to increase the range immensely if it is powerful enough. But no matter what with the monsters, they are no longer bound by the space and able to roam freely to any distance.