Castle Grayskull was a Masters of the Universe (books) read-along book written by John Braden and released in 1983 by Kid Stuff.
Description[]
The book tells the story of how Castle Grayskull was once the Hall of Wisdom, where the Council of Elders met and housed all the knowledge of the universe. One day a woman in snake-like armor appeared before the council and warned them that sometime in the future, Skeletor would come forth and try to seize their knowledge to enable his conquest of the universe. To keep him from doing so, the Elders compressed all their power and magic into a great orb, and turned the Hall of Wisdom itself into a terrifying fortress to protect it. This was how Castle Grayskull came to be. In the present, Man-at-Arms leads Prince Adam to the castle, where he gains the Power Sword and the ability to transform into He-Man.