Apart from Adi Parva of Mahabharata Sambhava Parva Chapters 71 & 72 - Shakuntala's History - Bala Kanda of Srimad Valmiki Ramayana narrates the History of Maharishi Kaushika who subsequently came to be known world over by his name Brahmarishi Vishwamitra in chapters 51 to 65.
Brahmarishi Vishwamitra had approached Emperor Dasharatha to send his sons Rama and Lakshmana with him to his hermitage to protect his yagnas from the rampages of the asuras. Intially Dashratha refuses, offering to accompany the sage himself and if required with his army, but Vishwamitra was adamant. On the advice of his preceptor Sage Vasishta, Dashratha allows his sons to accompany the sage. We all know the stories of the slaying of Tataka, Subahu and sending Marichi with his arrows to a far distant place. Most of us also know that it was during this expedition that Rama and Lakshmana acquired the knowledge of divine weapons Bala & Atibala.
After Vishwamitra finished his yagnas and austerities, Rama and Lakshmana accompany him to Mithila, on the way liberating Ahalya from an age old curse to re-unite her with her husband Sage Gautama and they reach Mithila.
There Raja Janaka welcomes Sage Vishmitra and his followers. Amongst the sages in present in Mithila was Maharishi Shatananda, the eldest son of Sage Gautama and Ahalya, and hearing from Sage Vishwamitra about his mother's liberation he welcomes Sri Rama and tells of the greatness of Brahmarishi Vishwamitra.
Indra the king of Devas thought that Raja Kaushika undertook penances to become as powerful as himself and deprive him of his throne and position. Apart from the attempt to seduce him through Menaka, which resulted in the birth of his daughter Shakuntala, Indra once disguised as a poor Brahmana deprived Vishwamitra of the fast breaking food, but by that time Vishwamitra was more in control of his senses and he quitely went back to perform austerities for another 10000 years. Indra tried to seduce Vishwamitra once again through Divine Nymph Rambha, but the infuriated sage cursed her to turn into a rock for 10000 years.
Brahmarishi Vishwamitra undertook severe austerities not to be equal to Shiva or Vishnu, nor was he interested in Indra's throne as Indra feared. Going back Vishwamitra was a noble king Kaushika. Once on his tours he came to the hermitage of Brahmarishi Vasishta and was entertained to a royal repast. This was possible by the Divine Cow known variously as Nandini, Kamadhenu, etc. gifted to him which could fulfill all the desires of its masters. Kauishka wanted it, but Vasishta would not part with it. In a pitched battle Vaishta easily vanquished the king and his army single handedly. The desire to own Kamadhenu prompted Kauishka to severe austerities and penances till he overcame his desires, conquered his senses and realized Brahman by becoming a Brahmarishi.