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.