At the beginning of this quest you will find a monk somewhere in Serkesh who has had a vision, which he has written down on a scroll. Talk to him and he will give you the scroll, telling of the demon who was defeated by a holy number. Keep exploring and you will find Mrs. Menthall, you tells you that her husband is missing.

Now venture down to level 25 where you will find a large maze, but this only contains a fake crystal. However if you like Potions of Surprise, you might like to find your way to the middle. Once you've either finished with the maze, or decided not to bother, continue to level 26 and 27, each containing a room with another fake crystal that turns into more Potions of Surprise.

Then on to level 28 where you will find a prison with the body of Mr. Menthall inside. Search the body and you will find a number of scrolls containing clues to what the Holy Number is, though be careful since some of them can be misleading. Also on that level will be a room with a demon statue, and a number dial in front of it (screenshot). You need to click on each dial, until you get the number 5 4 0 when looking at it from the entrance, which will also be 0 4 5 from the demons point of view, which is 12 times smaller than 540, meaning you have defeated the demon and you recieve the Crystal of Strength.