The books are part of a series written by Janny Wurtz called the Cycle of Fire. This was one of the earliest works of hers I ever read and it is also still one of my favorites.
It is not technically a fantasy book thought it has the trappings of the fantasy genre. You get this impression because of the iron-age technological level of the human locals and their feudal lifestyles.
There are Humans with latent psionic abilities which are synergistically bonded with a local but barely understood alien Sathid crystals which amplify their latent abilities a thousand-fold.
These psionic abilities resemble magical powers, so much so, no one in the world remembers how or why these powerful beings came to utilize their abilities or why they exist at all.
Adding to the mix are mysterious psychic alien creatures who, if you were a low-tech human, might mistake for magical beasts of incredible ferocity who are slowly taking over the planet.
The psychic aliens would have done so except for the efforts of the Stormwarden, who has powers over wind, weather and ice.
- The hidden truth is the humans are survivors of a starship seeking a means of repelling the aliens in a war taking place thousands of light year away who were originally looking for a weapon which would allow humans to resist the alien incursion into Human space.
This series is well worth the effort to find and read.