Gods Seek Brahma's Aid Against Dashagriva
Dashagriva, the ten-headed king of the rakshasas, is rampaging through the three worlds, protected by a boon that makes him invincible to gods and asuras. The gods, led by Agni, flee to Brahma for refuge — and learn that the solution has already been set in motion. Vishnu has already descended to earth, and the gods themselves must take birth among monkeys and bears to serve as his army.
The gods came to Brahma with a problem they could not solve.
Dashagriva — the ten-headed rakshasa king, son of Vishrava — was rampaging through the three worlds. He was immensely strong, protected by a boon Brahma himself had granted him long ago. No god could kill him. No asura could kill him. He was invincible by design, and he knew it. He oppressed all beings with injurious acts, and there was no one who could stop him.
The brahmarshis (sages who had realized Brahman), the siddhas (perfected beings), and the devarshis (divine sages) gathered together. They placed Agni, the god of fire, at their head and went to seek refuge with the creator.
Agni spoke directly: "O lord! Vishrava's son, Dashagriva, is immensely strong and cannot be killed, because you have earlier granted him a boon. The immensely strong one is oppressing all beings with injurious acts. O lord! Save us from him. There is no one else who can save us."
Brahma listened. Then he told them what they had not known.
"The gods and the asuras cannot defeat him in battle. What must be done to subdue him has already been ordained. For that purpose, at my request, the four-armed Vishnu, foremost among brave ones, has already descended on earth. He will accomplish this task."
But Vishnu could not do it alone. The grandfather laid out the plan: "With the masses of gods, all of you take births on earth. To aid Vishnu, all of you beget brave sons on monkeys and bears, strong and capable of assuming any form at will."
The gods agreed. All of them — the gods, the gandharvas (celestial musicians), and the serpents — descended happily to earth, taking on portions of themselves to be born in new forms.
Before they scattered, Brahma gave one more instruction. He summoned a gandharva lady named Dundubhi and told her what she must do so that the task of the gods might be accomplished. She listened, and then she took the form of Manthara in the world of men — a hunchbacked woman who would serve as the catalyst for everything that followed.
The foremost gods, with Shakra (Indra) at their head, begot sons on the chief women among monkeys and bears. These sons were like their fathers in fame and strength. They could shatter the peaks of mountains. Their weapons were shala and tala — trees and rocks. Their bodies were capable of withstanding the vajra (Indra's thunderbolt). Each possessed the strength of ten thousand elephants. They had the speed of the wind. Some lived where they wished; others lived in the forest. They were skilled in fighting and could assume strength and valour at will.
Having decreed everything, the illustrious creator of the worlds instructed Manthara about what she should do and how she should go about it. She carried it out with the speed of the mind — going here and there, bent on creating enmity.
The stage was set. The players were in place. The war that would end Dashagriva had not yet begun, but every piece of it was already moving. Aranyaka Parva, Chapter 557