The Sisters attacking Andy Dufresne.
The Sisters were a prison gang active in Shawshank State Prison in 1949. They were well-known for attacking weaker inmates, beating and raping them. According to Ellis Boyd Redding, they were not homosexual, but instead did their horrible deeds for sport. The Sisters' leader was Bogs Diamond, who took a liking to Andy Dufresne soon after his arrival at Shawshank.
The Sisters regularly attacked and sexually assaulted Dufresne, though this would end after Dufresne helped Captain Byron Hadley, who would paralyze Diamond in a horrific beating that would ultimately result in his transfer to a minimum security hospital upstate. The Sisters would disband, refusing to seek retribution against Dufresne out of fear of Hadley.