Watcher's Keep

Watcher's Keep is an ancient castle south of Amn. It has withstood age very well, and all in all, the keep is still in good shape.

History
It is told that the keep was built long ago as a prison for the most dangerous and evil creatures, of which the gods themselves wanted that they became imprisoned. For centuries it has been watched by a mysterious group of Helmites, the Knights of the Vigil, ordered by Helm to never let The Imprisoned One escape, or kill him because doing so would send him back to the Abyss which to him meant freedom and return to his home plane. To keep the wards holding The Imprisoned One intact, the leader of the Knights of the Vigil regularly sacrificed himself to keep wards in place. The wards kept The Imprisoned One inside the lowest level of the prison, a creature whose power allowed him to weaken the wards, thus making the sacrifices necessary. Eventually, the Knights of the Vigil lost their determination to sacrifice themselves, and began to spread news throughout the lands to lure some adventurers to the keep promising rewards for helping them.

Imprisoned One
The Imprisoned One was Demogorgon the demon prince and ruler of 88th layer of the Abyss.

In the game Baldur’s Gate II: Throne of Bhaal
The Player must fight through the five levels of this dungeon and withstand its traps and inhabitants. In the end he will meet the Imprisoned One and had to decide if he uses a scroll given to him by the knights to strengthen the wards in order to keep the demon imprisoned, or he can choose to fight and kill the demon which will however free Demorgorgon from his imprisonment, or the player can question if the player can leave the prison himself once the wards are strenghened.

If the player reads the scroll, he is trapped, but The Imprisoned One, posing as an innocent kept by enemies of Helm, can help the player escape. Once outside, the Kinghts of the Vigil runs to see what happened to the prisoner, then one of their earlier members who sacrificed herself appear and gives the player a scroll from Helm which will trap the fallen knights and The Imprisoned One. What happens then to the prison and who will guard it isn't revealed in the game.

Reference


Wachende Festung