The Forbidden Tower (Darkover‪)‬

In this book of the Darkover series, four challengers rise up to defy the matrix guardians.

Callista leaves the Keepers to marry Andrew Carr, and her twin sister, Ellemir, marries Damon Ridenow, creating a shared four-way bond. But the Keepers have tampered with Callista’s channels, and the foursome must navigate a tangled web of love, time travel, and prophetic dreams to challenge all the old traditions as they fight for a future of their choosing.