Vashishth is a Brahma Maanasa putr (ब्रह्ममानस पुत्र). Param sadhvi Arundhati is his wife. He is the Kulguru of the Ikshvaku dynasty rulers. He was called Vashishth as he had mastered the ‘Indriyas’ (इन्द्रिय ों क ’वश’ में करने वाला) through severe penance.
There lies an interesting story which depicts Vashishth’s and Viswamitr’s relation. Vishwamitr, before becoming a Rishi was a king. He was ruling Kanyakubj. One day, he went hunting. Tired of hunting, he and his soldiers reached Vashishth’s ashram. Vashishth received them cordially and treated them to a royal feast with the help of Kamadhenu – the celestial cow. Vishwamitr asked for the cow in return for 10,000 cows. Vashishth declined saying that the cow fulfils all the needs of the ashram. An angry Vishwamitr then tried to forcefully take the cow from the ashram. Kamadhenu created countless armies from its body, which destroyed Vishwamitra’s army. Enraged, Vishwamitr attacked Vashishth with divine weapons, all of which were reduced to ashes by Vashishth’s brahmadanda. Vishwamitr then realized the power of tapasya, carried out severe penance and became a Brahmarshi.
An Ikshvaku king, Kalmashapad got tired of hunting in the forest and goes towards Vashishth’s ashram. He sees Vashishth’s eldest son, Sage Shakti, walking in the opposite direction on the narrow path. The king orders Shakti to move aside. Shakti tells the king that Dharma is to give way to a Brahmin. Enraged, the king beats Shakti. Shakti curses the king to become a demon. Vishwamitra sees this and sends a rakshas to possess the king. Under the influence of the rakshas, the king serves human flesh to a Brahmin, who curses him to become a man eating rakshas.
A possessed Kalmashapad goes and eats Shakti and his 99 brothers. Vashishth, through his divya drishti, knows that Vishwamitr is the cause of this, but does not try to seek revenge. Keeping his sorrow to himself, and overcome with grief, Vashishth tries to end his life (though knowing it is a sin). He climbs Mount Meru and jumps down onto a rock, which turns into a soft cushion bed. Vashishth then tries to kill himself by fire, but the fire turns to ice. He then jumps into the ocean, but ocean washes him ashore with his waves. Vashishth binds himself with ropes and jumps into a raging river. The river frees him of his bonds (पाश) and saves him. From then it is called Vipasha. He then jumps into another river, which splits into 100 branches. Hence the river is known as ‘Shatadru(शतद्रु) (the ancient name for river Sutlej).
Realising that his efforts will not succeed, Vashishth returns back to his ashram. On his way back, he hears the chanting of Vedas from behind him. Turning back, he sees his son Shakti’s wife, Adrushyanti following him. She tells him that her son is chanting the Vedas from inside her womb. The love for his grandson makes Vashishth give up thoughts of ending his life. The boy is named Parashara (father of Veda Vyasa). One day, Kalmashapada came across Vashishth’s ashram and attacked him, Vashishth sprinkled water from his kamandal and lifted the curse. Vashishth then asked the king to rule justly and wisely, and granted a son to the childless king through niyoga. After this, Vishwamitr accepts Vashishth’s greatness and leaves his hostility towards Vashishth.
This is the story of Rishi Vashishth as narrated to Arjuna by the Gandharva king Angaravarna (अोंगारवर्ण )
(Source – Adiparv of Mahabharat)