A pilonidal cyst is a fluid-filled sac that occurs in the area at the top of the crease of the buttocks overlying the tailbone (sacrum). This cyst and the overlying skin in the area can become infected, forming a painful abscess. Pilonidal cysts are fluid-filled sacs that form at the top of the crease of the buttocks above the sacrum. 2  A painful abscess can form if the cyst and the overlying skin become infected.