One day, entities known as 'Gates' appeared in the world. A Gate is a rift that opens unexpectedly in the sky, allowing monsters to pour out and threaten humanity.

To prevent damage from these Gates, people began training individuals known as Sentinels, and academies were established.

Gates

Cracks that form in the sky without warning. The cause of their occurrence is still unknown, and currently, the best method is to close them as quickly as possible to minimize damage.

The scale of damage and the strength of monsters vary for each Gate, and they are ranked S-A-B-C-D-E-F in order of power.

Monsters pour out of the Gates to harm humans, and they are also ranked S-A-B-C-D-E-F based on their strength.

Each Gate has a 'Boss,' the strongest monster within it. Once the Boss is killed, the Gate closes.

Egris Academy

The most prestigious and largest academy in the world. All Sentinels at Egris live in dormitories. The student body is composed of various nationalities.

Sentinels are ranked S-A-B-C-D-E-F by strength. Higher ranks receive more benefits and better dormitory quality.

Egris Academy conducts written and practical exams every month. Familiars are allowed in practical exams, which involve defeating virtually generated monsters in the training arena.

A tournament where familiars can participate is held every two months.

Familiars

Assistants to Sentinels who can be summoned during the annual Summoning Ceremony. Though called assistants, it is essentially a master-servant relationship, and the familiar and summoner must share the same dormitory room.

Each student has two chances to summon during the ceremony; a familiar may or may not appear. Students can begin summoning from their second year.

Immediately after a familiar is summoned, their rank is measured using a crystal ball, categorized from S to F. If a power exceeding the existing system is detected, the crystal ball flickers with a red light. The crystal ball is indestructible.

After measurement, a contract can be signed if both the familiar and the summoner agree.