What is the darkest secret of Ravana?

What is the darkest secret of Ravana? Ravana desired to have intercourse with a woman against her will in a grove. Once Ravana came across a beautiful grove near Indraloka. There was a large pond in the grove filled with the waters of river Mandakini. It was the night of full moon. At that time the divine apsara Rambha came there to have a bath in the pond. Ravana noticed her from a distance. He swiftly went to the pond and seized her by the hand. He praised Rambha's beauty and desired to have intercourse with her. Rambha was shocked by Ravana's behaviour. She reminded him the fact that she was married to Kubera's son Nalakubara. Kubera was Ravana's half-brother and thus Ravana was Nalakubara's uncle. This made Rambha the daughter-in-law of Ravana. Rambha reminded this fact repeatedly to Ravana but the demon didn't pay any heed to her words. Ravana dragged Rambha with his arms and started to have intercourse with her. Rambha trembled with fear and repeatedly felt ashamed. She freed herself from Ravana's grasp and directly went to her husband Nalakubara. She told everything about Ravana's conduct to him. In a fit of anger, Nalakubara imposed a terrible curse upon Ravana. The curse was that Ravana's head would burst into seven pieces if he dared to engage in intercourse with a woman against her will in the future. Nalakubara's curse echoed through the three worlds and caused a terrible fear to Ravana. All the residents of the three worlds including Brahma and the gods were extremely delighted. Ravana refrained to have intercourse with women against their will in the future owing to the terrible curse of Nalakubara. This secret of Ravana is mentioned in Uttara Kanda the seventh book of Ramayana.