Nagpur rural police hasregistered a case against a Facebook user for allegedly obtaining obscene video of a 20-year-old woman and blackmailing her.The accused had been identified as Rahul Choudhary, a resident of Khalilabad, Uttar Pradesh, the police said.Choudhary befriended the woman on Facebook last year and soon they exchanged phone numbers and began to chat on WhatsApp, police said.After promising to marry the woman, the accused allegedly obtained her obscene videos through WhatsApp and began to blackmail her.A case of stalking and sexual harassment under IPC section 354, and also for extortion under section 384 was registered and further probe was on, police added.No arrest has been made yet.