The title refers to the crystal cave nestled inside of Galapas' cave outside Maridunum. (Yes, we're talking about a cave within a cave.) But it's not just the place where young Merlin has his first visions—it's also a jagged heap of foreshadowing.
Just in case you missed it, Merlin tells Cadal that he kinda sorta sees the crystal cave in visions of his death: "Sometimes I think it is death, and at other times it is birth or a gate of vision, or a dark limbo of sleep…I cannot tell. But some day I shall know," (II.11.67).
Okay, so the crystal cave can be symbolic of death, birth, sleep, or mystical vision. Thanks for narrowing that down, Merlin. But the main point? The main point is that the crystal cave is well out of our hero's comfort zone.
We don't get to see the end of Merlin's life in this book (there are still three more books in this series, after all), but we know it's out there. We also know that it has something to do with a wily woman and that darn cave. If you check out the Merlin stories that involve a temptress called Nimue or Vivienne (a.k.a. The Lady of the Lake), you'll be able to piece it together.