The elves in The Witcher are as mysterious as they are ancient, and there are those among them who spend their lengthy lives devoted to unlocking the mysteries of the universe. Crevan Espane aep Caoman Macha, known more readily to fans of the franchise as Avallac'h, was one such elf, and the wisdom and power he possessed far surpassed most of his kin. The sentient unicorns of the Aen Elle world called him the "Fox," perhaps because of his craftiness and cunning. Though he spent years teaching Cirilla of Cintra how to use her power as a source according to game canon, his motivations for doing so were beyond suspicious when all was said and done.
Avallac'h was one of the Aen Saevherne, an elven sage well-versed in numerous subjects, including the practice of magic. The Aen Saevherne were also known to possess prophetic abilities that allowed them to see, and perhaps even manipulate, the threads leading to the future. Avallac'h's prophecies first led Geralt of Rivia to him in Andrzej Sapkowski's novel, The Tower of the Swallow.
In the novel, the vampire Regis delivered a message to Geralt from the druids that led him to a cave, which he entered unarmed. There he encountered the elven sage, and Avallac'h proved his power of prophecy by identifying and naming Geralt and expressing knowledge of his quest to find Cirilla. Avallac'h then launched into a long-winded diatribe about the superiority of elves to humans. After the two philosophized for a while, he opened a portal to Tir ná Béa Arainne--the cemetery where Lara Dorren's memorial statue was housed.
There, Avallac'h revealed Ithlinne's Prophecy to Geralt, a detailed elven prediction about the eventual end of all things. A great ice age would pass through the realms, freezing and wiping out all humans in its path. The world would be born anew of the "Elder Blood," a seed that had been sown and would not sprout but burst into flame. The prophecy foretold the end of humankind and the rise of the elves.
Avallac'h was originally betrothed to Lara Dorren, and the two of them were meant to produce a powerful heir together, but Lara fell in love with a human man named Cregennan of Lod. Their love affair created an even deeper rift between elves and humans. When it was discovered she was pregnant, the elves felt betrayed--as if the humans had purposely stolen the Lara gene they had been working diligently to perfect in an effort to gain power. Lara died while giving birth to her daughter, Riannon, and her heirs became imbued with the Lara gene, gaining incredible power. Cirilla was one such heir, and before parting ways with Geralt, Avallac'h warned him not to pursue Ciri; otherwise, he would lose her forever.
In the final book in Sapkowski's Witcher series, The Lady of the Lake, Ciri accidentally teleported to the Aen Elle realm, and Avallac'h trapped her. After more than a week, he revealed that they wanted her to breed with their king, Auberon Muircetach, and create an heir that would return the Lara gene to them. At first, she refused, but when Avallac'h told her they would allow her to return home after producing the heir, she relented. After agreeing, she went out to ride, and while in the stables, Eredin approached and told her Avallac'h was lying to her and that no matter what she did, they would never let her leave Tir Na Lia.
Ciri did manage to escape Tir Na Lia, only to be pursued by the Wild Hunt. In The Witcher 3: Wild Hunt, CD Projekt Red deviated slightly from Sapkowski's lore, barely mentioning the fact that Avallac'h imprisoned Ciri in Tir Na Lia and practically forced her to have sex with his king to produce an heir. At some point, Ciri became Avallac'h's pupil, and it was he, according to game lore, that taught her to use and control her source power.
When Geralt met with the sorceress Keira Metz while searching for information about Ciri, he discovered that Ciri had been traveling with Avallac'h, but the two were separated. Throughout the game, Geralt's search eventually led him to discover a strange, mutated baby named Uma, which turned out to be a cursed Avallac'h. He also found evidence that Avallac'h had an obsessive fascination with Ciri's genealogy. In his lab, he met with an elven woman who claimed Avallac'h was only using Ciri for her genetics and once he got what he wanted, he would be free of her. Geralt could either choose to confront him about this or ignore it and trust the elven sage, but in his final battle with Eredin, King of the Wild Hunt, the elf's final words were a whispered warning of Avallac'h's betrayal of them both. Avallac'h leveled Geralt and Eredin against one another to steal Ciri away and open the World Gate.
The game suggests that despite her half-bred nature, Avallac'h may have had some romantic feelings for Ciri because of her resemblance to Lara Dorren. Drawings of Lara among his paperwork revealed this resemblance, but the elven woman claimed that the sage wanted nothing to do with her beyond her genetics. Avallac'h's true motivation in teaching Ciri to harness her power was an effort to open the World Gate and save his people, but did he develop romantic feelings for her over the course of their time together?
While it is impossible to say for certain, Avallac'h inevitably betrayed her by using her power to meet his own ends. Whether or not that was always his intention or if he regretted doing so remains unanswered, but there is no denying the bond that formed between them during their years of training together.