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Kratu (Sanskrit: क्रतु) (Sanskrit for "strength") was a rishi who appeared in two different ages. In the Swayanbhuva Manvantara, Kratu was a Prajapati and a very dear son of Lord Brahma. He was also the son-in-law of Prajapati Daksha. His wife was named Santhati. It is said that he had 60,000 children. They were named as included in the Valakhilyas. Kratu also had 2 sisters, Punya and Satyavati.
He is considered one among the seven great sages, the saptarishis believed to have originated from the mind of Brahma. As per another legend, he is believed to have been born from his father's left eye. He was married to Sannathi and the pair had sixty thousand children, called Balakhilyas, who were of the size of a thumb and resided on the bank of rivers. There were once insulted by Indra, the king of celestial deities. By the power of their penance, they were able to please Shiva, who gave him a boon that they would be able to produce a bird which would pull out nectar from Indra.
The head of the sacrifice was none other than Prajapati Daksha, who was nursing a grudge against Rudra or for the simple reason that his daughter Sati married Shiva against his wishes. He did not send even an invitation to Shiva, who was his son-in-law.
Sati went to the sacrifice uninvited anyway, and was terribly insulted for marrying a beggar. She was not able to listen to the insults hurled at her by her father and her sisters. She committed suicide by jumping into the sacrificial fire.
Hearing the news, Lord Shiva became mad with rage. He pulled out a lock of hair from his head and threw it down, invoking the Spirit of Veerabhadra or Bhairava. He ordered Veerebhadra to go with his followers and destroy everybody who attended the Yagna, including Prajapati Daksha. Accordingly, they went and be-headed Daksha. Then they started punishing each and every god who attended the holy sacrifice. They knocked out the teeth of Pushan, took out the eyes of Bhaga, and severed the two testicles of Kratu.
All the gods went to Shiva and begged for his mercy. Shiva said that they should all serve under Him as animals for some time; only then would they will regain their original status. All the gods agreed to this. Shiva then restored the teeth, eyes and testicles of the different Gods. Daksha was also revived by placing the head of a goat on his neck. After his testicles were restored, Kratu married Sannati (Santathi), the daughter of Daksha. They were all pygmy sages no bigger than the joint of the thumb. They were all chaste and pious. All were Brahmacharis and students of Vedas. (References: Maitreya Samhita 4-2-12 And Brahmanda Purana)
Rishi Kratu was again born in the Vaivaswata Manvantara because of Lord Shiva’s boon. In this Manvantara he had no family. It is reported that he was born from the hand of Lord Brahma, whereas other Rishis are described as having been born from other parts of the body of Lord Brahma. (Reference –Bhagavat). As he had no family and no children, Kratu adopted Agastya’s son, Idhvaaha. Kratu is considered as one of the Bhargavas. In Matsya Purana it is said that his mother’s name was Poulomi. He is also considered as one of the Viswa Devas.