We had read from various sources about Ravana – the greatest Asura king who took control over devatas- Yakshas, Gandharvas, etc., But nowhere this is stated that even Ravan was unable to win over a clan whose mighty power had terrified all the other races.
Nivatakavachas, an Asur demon race with supernatural power. They were counted to be around 30million in number. They reside in cities that float on seas also underwater. They had extremely powerful magic and warfare techniques with ultra-powerful weaponry. Lord Indra, who tried to invade this clan had failed miserably several times. The most frightened and powerful demon race that existed on the face of earth were the Nivatakavachas.
When Ravana successfully defeated the Land of Death, the city of Yama, his ministers had celebrated his victory. Though severely wounded, he came out of the city alive which was unable to any warrior who faced Yama. With an obsession, he entered the Bhogavati city and defeated the Vasuki (the Nagas). Then his greed for power fell on Nivatakavachas who never tasted a defeat. When he challenged Nivatakavachas for a battle, they enthusiastically accepted as they haven’t been in a battle for so long time.
Everyone frightened to raise a war with Nivatakavachas, as they possessed unbeatable power and magic. So the invincible clan fought with Ravana for over a year with neither side yielding to the other. Then Lord Brahma came to stop this battle as the world may end due to the terror created by them. Then both the parties sword friendship before the fire.
After this Ravan resided in the capital of Nivatakavachas for over a year where he learned 101 tricks in black-magic. And this is the proof that Nivatakavachas were never defeated by Ravan, the Demon King. Nivatakavachas could only be defeated by “Rudra Weapon” which Ravan didn’t have.
This weapon of Lord Rudra was only possessed by Arjun in the Dwapara Yuga. Lord Indra asked Arjuna to destroy the Nivatakavachas and so they were into battle with each other. Then finally, the whole race with their cities were also smashed into ashes. As this race disappeared from the face of the earth, the ultra-advanced submarine warfare technology also got destroyed and was never compiled again.