Shukracharya was the guru of Asuras. He knew the Mritasanjivani Mantra with which he could revive the dead. He used the mantra to help Asuras and this led to the defeat of Devas. The story of Kacha, son of Brihaspati and a deva, is mentioned in the Adi Parva of the Mahabharata.
To help Devas, Kacha volunteered to go to Shukracharya to learn the mantra.
Kacha became the student of Shukracharya in spite of him being a Deva. Asuras were not happy with the decision of Shukracharya.
Devayani, the daughter of Shukracharya, fell in love with Kacha.
Kacha was killed on two occasions by the Asuras, who disliked his presence in the ashram of Shukracharya. However, for the sake of his daughter, Shukracharya used the Mritasanjivani Mantra and revived him.
Third time Asuras killed Kacha and burned him. They then mixed the ashes in the wine of Shukracharya.
Shukracharya soon realized that Kacha was in his stomach. However, this time if he revived Kacha, he would come out burst opening the stomach and the sage would die. If he did not revive Kacha, Devayani would die.
To come out of the tricky situation, Shukracharya taught Kacha the Mritasanjivani Mantra.
Soon Kacha emerged from the stomach of Shukracharya. He then used the mantra to revive his Guru.