Warning: Spoilers for Witcher: Nightmare Of The Wolf
The Witcher: Nightmare Of The Wolf features a variety of Witcher characters viewers may be familiar with - including a surprise appearance from one the franchise's most iconic characters at its very end. The Witcher: Nightmare Of The Wolf is set a fair few decades before the plots of the books, games, and show - meaning that a large number of recognizable characters simply couldn't appear in the anime film, as they were born years after the events that take place in it. However, the nature of the Witcher transformation means characters in this profession don't age at the same rate regular people do; a detail that allows Vesemir to star in a plot that involves serious combat despite being around eighty at the time of the battles.
The Witcher: Nightmare Of The Wolf covers some relatively untouched ground, as the life of the Witcher mentor is something that isn't covered in the most detail in any of the franchise's different media, especially when it comes to Vesemir's life prior to training the series' major protagonist Geralt. The film does an apt job of shedding light on this time, showing Vesemir in childhood, his decision to become a Witcher, and his shift from being relatively uncaring towards new recruits to becoming a last real mentor of Witchers, as a result of being one of few survivors of an attempt to cull them. After the battle, the remaining Witchers alive appear to be Vesemir and the new recruits that had been sent away from the battlefield - and, sensing these recruits still needed his help, Vesemir jumps into the role he once feared, gruffly establishing himself as the new guardian of these children.
As it turns out, this decision would play a vital role in the larger plot of the franchise, as one of the new recruits - a bald child who responds to Vesemir's call to action with "but they hate us" - is promptly name-dropped as being Geralt, the main character of the majority of Witcher media. Similar to Vesemir, prior to this little had ever been revealed about Geralt of Rivia's childhood, or his early days of Witcherhood, and so this serves to bridge a gap of sorts in the collective history of the franchise - although it likely wouldn't serve as canon for the books, games or show.
Geralt's introduction at the end of the movie serves as a clever development of Vesemir's character arc, too, as the older Witcher establishes early on in Nightmare Of The Wolf that all he really wants is a family, and quickly grows frustrated with the new recruits, as they serve as a reminder of the children he can never have due to Witcher transformation. At the end of the plot, however, Vesemir appears to have made a kind of peace with his lot in life, and instead takes on the likes of Geralt as his new wards - effectively making them the family he has sought all this time.
The Witcher: Nightmare Of The Wolf could easily have taken the focus off of Vesemir by introducing Geralt at any point in time, and so it serves the film well that it manages to make this plot point more of an effective way to link the histories of two important characters, rather than undermine Vesemir's one moment in the spotlight. With the franchise growing ever larger, there is hopefully an ever-growing potential for more installments like The Witcher: Nightmare Of The Wolf, that flesh out the world of The Continent and the characters within it, outside of the frame of the universe's main story.