This keep is a huge dungeon which you may choose to explore either during Shadows of Amn or during Throne of Bhaal. However, I warn you – you will meet some very tough enemies, so it is recommended that you wait until you have a very strong and experienced party.
Travel to the Watcher's Keep area, and you will meet a group of Priests of Helm, the Knights of the Vigil, who are charged with keeping guard over the keep, where Helm himself once locked up an ancient enemy. But the old wards are breaking apart and they need you to enter the keep, find your way down to the lowest level, where The Imprisoned One is locked up. Here you must read a scroll to seal off the keep once more.
And of course there is a little twist to the story at this point.
You begin your dungeon crawl in the Watcher's Keep Outside Area!