Janamejaya invaded Taksasila (Ajanta), and he decided to avenge the unlawful curse upon his great father, Maharaja Pariksit
SB Canto 1
King Parīkṣit married the daughter of King Uttara and begot four sons, headed by Mahārāja Janamejaya.
Mahārāja Uttara was the son of Virāṭa and maternal uncle of Mahārāja Parīkṣit. Irāvatī, being the daughter of Mahārāja Uttara, was the cousin-sister of Mahārāja Parīkṣit, but cousin-brothers and -sisters were allowed to get married if they did not belong to the same gotra, or family. In the Vedic system of marriage, the importance of the gotra, or family, was stressed. Arjuna also married Subhadrā, although she was his maternal cousin-sister.
Janamejaya: One of the rājarṣi kings and the famous son of Mahārāja Parīkṣit. His mother's name was Irāvatī, or according to some, Mādravatī. Mahārāja Janamejaya begot two sons of the names Jñātānīka and Śaṅkukarṇa. He celebrated several sacrifices in the Kurukṣetra pilgrimage site, and he had three younger brothers named Śrutasena, Ugrasena and Bhīmasena II. He invaded Takṣaśilā (Ajanta), and he decided to avenge the unlawful curse upon his great father, Mahārāja Parīkṣit. He performed a great sacrifice called Sarpa-yajña, to kill the race of serpents, including the takṣaka, which had bitten his father to death. On request from many influential demigods and sages, he had to change his decision to kill the race of snakes, but despite stopping the sacrifice, he satisfied everyone concerned in the sacrifice by rewarding them properly. In the ceremony, Mahāmuni Vyāsadeva also was present, and he personally narrated the history of the Battle of Kurukṣetra before the King. Later on by the order of Vyāsadeva, his disciple Vaiśampāyana narrated before the King the subject matter of Mahābhārata. He was much affected by his great father's untimely death and was very anxious to see him again, and he expressed his desire before the great sage Vyāsadeva. Vyāsadeva also fulfilled his desire. His father was present before him, and he worshiped both his father and Vyāsadeva with great respect and pomp. Being fully satisfied, he most munificently gave charities to the brāhmaṇas present at the sacrifice.