The Witcher series, Nightmare of the Wolf tells the story of how a young Vesemir is indirectly responsible for Kaer Morhen’s dwindling numbers.
The Nightmare of the Wolf follows Vesemir as he contends with human and non-human threats in the lands surrounding the witchers’ guild, Kaer Morhen. As he and sorceress Tetra Gilcrest investigate where exactly the monsters in the kingdom Kaedwen are coming from, they discover that a mage is using a witcher-specific brand of alchemy to mutate monsters together into significantly more powerful crossbreeds. That said, not all of these crossbreeds are horrifying monsters, and Vesemir decides to spare an elven one despite growing vitriol surrounding non-human creatures.
It is this exact act of mercy that sends an avalanche of devastating events in motion, one that ultimately leads to the near-annihilation of the witchers at Kaer Morhen. By sparing a being that is made up of two of the human population’s most contentious enemies, Vesemir inadvertently creates the perfect opportunity for Tetra to build enough hatred for the witchers and recruit an army to fight them. Unfortunately enough, Vesemir’s decision to save one is what kills so many at Kaer Morhen, leaving the School of the Wolf with five survivors.
Throughout Nightmare of the Wolf, sorceress Tetra shows extreme contempt for the witchers, looking for any shred of evidence that may tie them to the monsters terrorizing Kaedwen. When she and Vesemir discover that mutation-based alchemy has been used to create these monsters, she takes this as definitive proof of their involvement. This alchemy is known only by the witcher mages and is directly tied to the witchers' origins. Her campaign only comes to its head though because of Vesemir - by allowing the crossbreed made from witcher alchemy free, Vesemir cements the witchers’ guilt in her mind. This drives her to the point where she elevates her mission from investigation to siege, thus beginning the battle at Kaer Morhen. Had Vesemir decided against sparing the child, Tetra’s anger may not have built to the peak that it did, nor would there be enough evidence of witchers supposedly conniving with monsters for Tetra to use. While he’s not directly responsible for bringing the army of humans and monsters to the guild, it was his actions that spurred Tetra to do so.
The most tragic event in Nightmare of the Wolf, however, is when Vesemir’s decision is used to manipulate another extremely powerful mutated elf mage named Kitsu. Kitsu considers the child to be her own, and Tetra uses that to manipulate her into believing the witcher is responsible for killing, not saving, the child. This ultimately becomes the reason why she s Tetra, giving the sorceress access to the vicious monsters in the battle. If Vesemir had not angered Tetra nor set the child free, Kitsu wouldn't have been tricked, and Tetra’s army wouldn’t have been as overwhelming. His choice is what enabled Tetra to be so complete in her destruction. Additionally, without Kitsu’s illusion magic, Tetra couldn't trick Vesemir into killing the mage and thus couldn’t destroy the witchers’ ability to create new .
Ironically and tragically, Vesemir’s mercy is what enabled Tetra to kill so many witchers and to kill the future of the School of the Wolf. Geralt's mentor Vesemir is slated to appear in season 2 of The Witcher live series, it is yet to be seen how he and these integral events in Nightmare of the Wolf will play into the story.
