During their exile in the forests, the Pāṇḍavas (Pandavas) heard the legend of an ancient Rājarṣi (Rajarshi or a king with the attributes of a seer or sage) called Indradyumna, from Sage Mārkaṇḍeya (Markandeya). However, in the Mahābhārata (Mahabharata) it is not mentioned in which age he was born, or to what vaṁsa (vamsa) he belonged.
The Pāṇḍava (Pandava) and other sages asked the long-living Sage Mārkaṇḍeya (Markandeya) whether there had been any other person who is older than the sage himself. In response, Mārkaṇḍeya (Markandeya) said that king Indradyumna was more ancient than himself. The sage himself acknowledged the fact.

Indradyumna lived for a long time in heaven due to his virtues. After this, the fruits of his virtuous deed got exhausted, and he fell back to the earthly abode–

asti khalu rājarṣirindradyumno nāma kṣhīṇapuṇyastridivat pracyutaḥ.

Coming back to earth., king Indradyumna, with a sad heart, started thinking that all his virtuous deeds had been exhausted. Nobody as ancient as himself was living now, that he would remember his good deeds. Thinking so, he went to the ancient sage Markaṇḍya and asked him, “Do you remember my good deeds?” Markaṇḍeya was an ascetic recluse, and he kept travelling from one pilgrimmage spot to another. He could not remember worldly matters about the king̍s achievements. The king asked him, “Is there anybody who is more senior to you?” Markaṇḍeya took him to an ancient owl, living in Himālaya, called Prāvarakarṇa. He was senior to Markaṇḍeya, but he also could not remember the king. However, on his suggestion, the king went to a heron named Nāḍijangha, living in the lake ̍Indradyumna̍. This heron was more ancient than the owl, but even he could not remember the king. The king asked him, “Is there anybody who is more advanced in age than yourself?” He said, “In this lake, lives a tortoise called Akūpāra. He is senior to me. You should go to him.” The king met Akūpāra and asked, “Do you remember king Indradyumna?”. Considering a lot, Akūpāra answered, “Why shouldn̍t I remember king Indradyumna? He performed thousands of yajña(s), and gave in donation a number of cows. By the grazing of those cows, this Indradyumna lake, where I am living, had been created.” As Akūpāra praised the good deeds of Indradyumna, the heavenly chariot came down to take Indradyumna back. Indradyumna again returned to the heavenly abode.