The Viper School lies somewhere within the Empire of Nilfgaard. At some point it fell into ruins but the events that have caused its downfall are not mentioned by Letho in his account to Geralt in the Epilogue of The Witcher 2: Assassins of Kings. Students or adherents of this school include:
It is also possible that the mysterious assassin from the end of The Witcher may have been one of the two unnamed members, rather than a third.
What we do know about the school is that its members have become outcasts within the Nilfgaardian Empire since the school's downfall. Before its destruction it had a huge collection of books about the Wild Hunt.
It is interesting to add that the school might have differed from other witcher schools in its swordsmanship training, as it seems that almost all witchers belonging to the Viper School (except Letho) used two short swords in combat as their weapons of choice, as opposed to the traditional usage of two-handed swords preferred by other schools. However, all witchers known to use short blades were Kingslayers, and it is possible that they chose to forfeit using their long swords for the more compact counterparts to better serve the purpose of assassinating kings of the North. Whatever the truth, these witchers possessed exceptional skill in wielding short blades, and their skill in swordsmanship was rivaled only by that of Geralt of Rivia.
According to Letho, School of Viper has a lot of information including books about the Wild Hunt.