The Crystal Cave is a fantasy novel written by Mary Stewart in 1970. The book mainly focuses on the character of Merlin, from the Arthurian legends, before he became the legendary magician. The Romans have recently left Britain and the kingdom is divided. Merlin is the son of a Welsh princess and regularly has clairvoyant visions. Merlin is trained by a hermit who teaches him to use his powers and Merlin later finds himself at the court of Ambrosius Aurelianus, who wants to invade Britain and take it from Vortigern, a Saxon usurper. Merlin returns to Britain but is captured by Vortigern and taken to a fortress which keeps collapsing because of caves underneath it, unknown to Vortigern. Ambrosius invades Britain and defeats Vortigern. It is revealed that Ambrosius is Merlin's father. Ambrosius later dies and his brother Uther becomes King of Britain. Uther later enters a tryst with his lover and Merlin realizes this will lead to the birth of King Arthur.
The book was written by the author to delve into the Arthurian legend as have many other authors and so that the author can give her own twist on the origin story of Merlin.
The book was received well by critics and fans alike. In fact, the Daily Mail has commented on the book stating that it was "vivid, enthralling, absolutely first-class". The novel was also turned into a television series by the BBC in the 1990s.