The Cycle of Fire

The Cycle of Fire is an 1980s Fantasy trilogy, with a Science Fiction backstory, written by Janny Wurts.

The world of Keithland looks like standard issue High Fantasy, with a feudal society locked in Medieval Stasis under constant attack from a demon lord lairing in the far north; but it becomes apparent that the demons are actually psychic aliens, and the world a lost colony. Humanity, the only race with no significant psychic ability, was on the losing side of a genocidal war, so a ship was sent out to search for a countermeasure. That mission failed when the prisoners on board escaped, forcing it to land an an uninhabited world - the world of the story.

The surviving crew deliberately reverted to a medieval society, concealing all evidence of their true origins, so that the aliens who now posed as demons would not suspect that the ship had survived intact. A secret inner circle in the priesthood retained memory of the truth, but could only passively wait for the demons to go away. Fortunately, the ship's computer had also survived. Posing as Tamlin of the Vaere, one of The Fair Folk, it stayed true to its assigned mission. Unfortunately, all the civilised societies of Keithland view Tamlin and its agents with deep suspicion, not without reason.

At the start of the trilogy, Tamlin's scheme is the creation of sorcerors through a psychic bond with the power-amplifying living crystals called Sathid, but this is no partnership. The would-be sorcerer and their crystal fight a mental duel for dominance. Victory reduces the Sathid to nothing more than a battery; defeat means death. Worse, to gain elemental mastery, the sorcerer must experience illusory death by that element at the climax of the mental duel. Repeating this process a second time is much harder, but results in much greater power. So far, no one has survived a third repetition, nor have they been able to master fire without sacrificing all empathy. The last Firelord was nearly as destructive as the demons.

The three books are:

Stormwarden: The orphan Jaric discovers he is the son of the hated Firelord when he is called to fight the demons in his father's place, while a brother and sister from a remote village stumble into the middle of a conflict between the Stormwarden and Tathagres, agent of the demons.
Keeper of the Keys: Jaric searches for a safer alternative to sorcery while being hunted by demons.
Shadowfane: Jaric becomes a Firelord, like his father, but the demons' mistreatment of their minions has unleashed a greater evil.

Tropes used in The Cycle of Fire include:

Exclusively Evil: The demon races. Partially subverted by the Llondelei, believed to be as evil as the other aliens, but actually all good.
Big Bad: First Demonlord Scait, then the Morrigierj.
Breath Weapon: The Mharg demons have breath so poisonous that the affected areas remain barren wastelands for decades afterwards.
Call to Adventure: In the form of a spell (specifically called a "geas") placed upon Ivain Firelord by the Stormwarden Anskiere, that would force him to come to Anskiere's aid when it is invoked; the spell would also pass on to Ivain's children in the event of his death.
Detect Evil: There is a field projected around the Free Isles by ancient technology from the humans' wrecked starship that detects any demons that cross its border.
The Dragon: Successively Tathagres and Maelgrim Darkdreamer.
Dream Weaver: Taen Dreamweaver, naturally, and her brother, Maelgrim Darkdreamer.
Elemental Powers: And high-end, too. The Stormwarden creates winds strong enough to melt rock by frictional heating alone, while Jaric goes nuclear.
The Fair Folk: Most humans believe the Vaere to be this.
Fighting Down Memory Lane: Used during the various attempted Mind Rape scenes.
Functional Magic: The sorcerers use a combination of Elemental Powers, Psychic Powers, and force magic. Wizards use rule magic.
Human Popsicle: The Stormwarden freezes himself in ice to trap some demons.
Lost Colony: The setting, unknown to any of the characters until the very last pages.
Mind Rape: Inflicted by the demons on their human pawns.
Necromancer: Maelgrim Darkdreamer kills everyone in a northern country with nightmares in a single day, then animates all their corpses.
Nigh Invulnerable: The only known way of killing a Morrigierj is dropping it into a star.
The Power of Love: Gives Jaric the strength to survive his tortures.
Psychic Powers: Empathy and prescience.
Refusal of the Call: Jaric spends much of the first book resisting the call, and of the second looking for another way of answering it than his father's.
The Call Knows Where You Live: It doesn't involve a Doomed Hometown, but when Anskiere invokes the geas, Ivain has already died so The Call gets rerouted to his son Jaric. And because of the way the geas works, it's more like "The Call Will Cause You Massive Physical Pain Until You Answer."
Training from Hell: To master fire, Jaric must experience being burned alive, endlessly.
Science Fantasy: Magic and aliens.
Shapeshifting: The Karas demons can imitate anyone, and steal their memories too, but they have to eat their victim to complete the transformation.
Scry vs. Scry: Jaric and the Morrigierj both use prescience during their duel, but the Morrigierj prevents him from seeing the positive outcomes.
Squishy Wizard: The wizards of Mhored Kara are all described as being emaciated, having spent most of their lives in study and forsaking most physical work.
With Great Power Comes Great Insanity: The usual cost of mastering fire.
Younger Than They Look: During Taen's training by the Vaere, she was put into a life-support capsule inside the hidden facility beneath their island. She aged several years while in the capsule, but her leg (crippled in a childhood accident) was healed by the Vaere's technology. (It's a little ambiguous since the aging is due to time dilation and she was aware of time passing in a virtual reality while in the capsule, so she is in some ways exactly as old as she appears.)