Cantankerus Nott was a pure-blood wizard and a zealous advocate for pure-blood supremacy. He is notable primarily as the anonymous author of the Pure-Blood Directory, a controversial publication from the 1930s that sought to definitively list the British wizarding families he considered to be “truly” pure-blooded. The families listed in his work became known as the Sacred Twenty-Eight, and his biased definitions had a lasting impact on the social hierarchy and prejudice within the wizarding world. All known information about him comes from supplementary materials written by J.K. Rowling. (Pottermore)
A member of the notoriously conservative Nott family, Cantankerus Nott anonymously published the Pure-Blood Directory sometime in the early 1930s. The book's stated goal was to help preserve the purity of wizard bloodlines by identifying the families that had remained “untainted.” (Pottermore) Nott's methodology was widely criticized as flawed and based on his personal prejudices. He arbitrarily excluded families for reasons he deemed sufficient to suspect “tainted blood.” For instance, the Potter family was omitted because their common Muggle surname led Nott to believe they originated from impure stock. He was forced to include the Weasley family due to their demonstrable pure-blood ancestry, but he noted their affinity for Muggles and branded them as blood traitors, suggesting they were as “bad as Muggle-borns.” (Pottermore) The publication of the directory sparked outrage among many of the excluded families, who argued that their ancestors simply had Muggle-sounding names or that no family could prove an entirely pure lineage without some degree of self-deception. Despite the controversy, the twenty-eight families listed in the directory came to be known as the Sacred Twenty-Eight, a term that became ingrained in the lexicon of pure-blood elitists. (Pottermore)
No physical description of Cantankerus Nott exists. His personality, as inferred from his work and his given name, was elitist, prejudiced, and argumentative. His creation of the Pure-Blood Directory reveals a deep-seated belief in pure-blood superiority and a contempt for anyone with Muggle ancestry or sympathies. His decision to publish anonymously suggests a possible awareness of the controversy his work would generate, though it does not imply any lack of conviction in his beliefs. (Pottermore)
There is no specific information regarding Cantankerus Nott's magical prowess, wand, or other abilities. His work on the Pure-Blood Directory suggests he possessed skills in writing and historical research, albeit applied with extreme bias.
Cantankerus Nott is not mentioned in any of the seven original Harry Potter novels. His existence, history, and the creation of the Pure-Blood Directory were established by J.K. Rowling in an essay titled “The Sacred Twenty-Eight,” originally published on the Pottermore website. This character and his work serve to provide a canonical origin for the concept of the Sacred Twenty-Eight and to further develop the historical context of blood purity prejudice in the wizarding world.