>>719674198
The king then had him sealed away inside of a "black egg", a chamber created from more void substance, and sealed that egg with a powerful spell that drew on the vital energies of a trio of collaborators which were placed into a permanent and immortal sleep, and themselves hidden away at the far reaches of the kingdom.
However, he made a mistake. The fondness he showed to the hollow knight during the preparation resulted in the hollow knight gaining enough emotion that the radiance was able to slowly eat away at his will, and eventually begin to influence the real world once more. This was somewhat limited by the black egg, but the egg also meant that nobody could get back in to deal with the hollow knight.
In the basic ending, the knight, your character, breaches the black egg, kills the hollow knight, and replaces him as vessel, taking the radiance within himself. Given that not even the ostensibly perfect vessel was able to contain her forever, we can anticipate that he too would eventually fail.
In the true ending, the hollow knight regains his lost memories of his origins and reconciles the tragedy of his creation with the potential of his current existence, and enters the mind of the hollow knight to challenge and destroy the radiance directly in her own dream realm. In the process, he sheds his physical form, and after the radiance is destroyed, he and the hollow knight return to the abyss in spirit, now at peace.