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Video games are considered canon unless they contradict content in some other Forgotten Realms publication.
The keep was built long before the 14th century DR as a prison for the most dangerous and evil creatures, which the gods themselves wanted imprisoned. For centuries, it was watched by a mysterious group of Helmites, the Knights of the Vigil, ordered by Helm to never let the Imprisoned One escape, or be killed, because doing so would send him back to the Abyss and freedom in his home plane.
To keep the wards holding the Imprisoned One intact, the leader of the Knights of the Vigil regularly sacrificed one of their own. The wards kept the Imprisoned One inside the lowest level of the prison, but the creature's power allowed him to weaken the wards, thus making the sacrifices necessary.
Eventually, however, the Knights of the Vigil lost their determination to sacrifice themselves, and began to spread rumors throughout the land to lure adventurers to the keep, promising rewards for helping them.
Gorion's Ward and companions came to the Keep and had to fight through five levels of the dungeon and withstand its traps and inhabitants. In the end, they met the Imprisoned One and had to decide whether to use a scroll given to them by the Knights to strengthen the wards and keep the demon imprisoned, and risk imprisonment themselves; or choose to fight and kill the demon, which would however free him from his imprisonment.
If they read the scroll, they were trapped, but the Imprisoned One, posing as an innocent kept prisoner by enemies of Helm, could help them escape. Once outside, the Knights of the Vigil ran to see what had happened to the prisoner, then one of their earlier members who sacrificed herself appeared and gave the Gorion's Ward a scroll from Helm, which would trap both the fallen knights and the Imprisoned One.