The Black Guardian is the hereditary title and form that chosen Fafnir Knights must undertake and become. Undertaking this form is what makes it possible to seal away the Yggdrasil Core of High Lagaard.
The Ritual[]
Once the Knight chosen by the Fafnir overcomes the trials of Ginnungagap, they are deemed worthy of inheriting the title of Black Guardian. When they pass through the Door of Boundaries, they will be shut off from the rest of the world as the ruins transfers the power of the Black Guardian to the new successor. The former Black Guardian's spirit will pass on, and the current one will remain in Ginnungagap, maintaining the seal over the Yggdrasil Core until their life force is drained. Only then will a new Knight be chosen by the Fafnir and be directed to Ginnungagap to renew the seal, staring the cycle anew.
The Anomaly[]
The cycle was broken by Bertrand de Gervaise and Princess Violetta. As Bertrand had to leave to attend to his ill sister and mourn his mother's passing, Violetta decided to inherit the power of the Black Guardian instead. However, she was not as strong as the other Fafnir Knights before her, and she had trouble maintaining the seal over the Core. This led to the weakening of the seal and the collapse of Ginnungagap's upper floors, leading to their decrepit state the player sees.
When the protagonist undergoes the ritual at Ginnungagap, the system also recognizes Bertrand as a valid successor and mistakenly begins transferring the power to him. The party has to interrupt the ritual and defeat the Demi-Fafnir so that the protagonist can safely inherit the role of Black Guardian. However, due to the extensive damage that Ginnungagap has sustained, the ritual cannot be continued, and the protagonist becomes the last to inherit the power of the Black Guardians. The original Black Guardian is disappointed by this outcome, but directs the party to meet with the Overlord to find a new way to resist the Calamity before the seal becomes undone.