I would say the most fucked up thing I read about was a sex slave in Borneo named "pony".
She was kept in a compound where she was chained to the wall and had sex with over 400 men when she was only 6 years old. She was covered in sores and mosquito bites when she was rescued by policemen armed with AK47s. If you think I'm making this up click here
Most Helpful Girl
the Fritzl case was already mentioned, so check this out :
Four 17 year olds kidnap a 16 year old, raped her, beat her with various objects, introduced foreign objects including a light bulb into her vagina, made her eat cockroaches and drink her own urine, inserted fireworks into her anus and set them off, forced her to masturbate, cut her nipple with pliers, dropped dumbbells onto her stomach, and burned her with cigarettes and lighters. One day, they beat her with an iron barbell, poured lighter fluid on her legs, arms, face and stomach, and set her on fire. She died later that day of shock. The torturers hid her corpse in a 55-gallon drum filled with cement and disposed her body.