The Fellowship of the Basilisk was a group of mainly Slytherin students at Hogwarts School of Witchcraft and Wizardry. The group was founded by Severus Snape, Narcissa Black and Regulus Black, however Severus was looked on as its paramount leader. In addition to the three founders, the group was also joined by Augustus Greengrass, Mary MacDonald, Evan Rosier, Penelope Burke and Jennifer Pratt.

The Fellowship's mission consisted of disrupting the power-base of the Knights of Walpurgis; which consisted of many future Death Eaters, Combat the Marauders' harassment of Slytherins and other people they deemed 'undesirable', and protect Muggle-born students from hexing by Blood Purists.

The Fellowship engaged in a hexing and prank war with the Marauders in 1976; the loser had to admit defeat and be emasculated in front of the entire Great Hall. This conflict lasted two months and came to a head when Narcissa Black was attacked and seriously injured by Sirius Black; hexing her into a statue, giving her a concussion and breaking her leg. Minerva McGonagall forced the Marauders to surrender and the Marauders disbanded as a group. The famed "Marauder's Map" was taken as a spoil of war.