The son born to Vishwamitra with Yayati’s wife Madhavi, as per Mahabharata Udyogaparva Chapter 119 verse 18. (see Galava)

A story told by the sage to Markandeya to the Pandavas. Ashtaka conducted an ashwamedha yajna, which many kings attended. Among them were his brothers Pratardana,  Vasumanas and Shibi. After the yajna, Ashtaka conducted an aerial trip with his brothers. On the way, they met Narada and took him along. Narada was asked that if only 4 of them could ascend to the heavens, who would have to step down. He replied that it would be Ashtaka. He explained that when he had stayed with Ashtaka, he had seen many cows and when he asked about them, Ashtaka proudly said that they were the ones he was going to donate. This was the reason why he couldn’t ascend.

When Narada was further asked if only three people could ascend who would have to step down, he said that it was Pratardana. He explained that once, when they were on a chariot together, 4 brahmins asked him for a horse each, one after the other. He asked them to wait for a while, but they refused. He gave three of them his spare horses, and when the fourth brahmin insisted on one too, he gave away the horse that was drawing his chariot, and started pulling it himself, while blaming the brahmins’ impatience. This was a black mark on him.

Narada was then asked that if only 2 could go to heaven, who would have to step down. He replied that it would be Vasumanas. On the 3 occasions that Narada had visited him, Vasumanas would talk about how wonderful his flower chariot was, to which Narada would agree. The brahmins present there would also concur and Vasumanas would feel proud. This made Vasumanas guilty.

When Narada was further asked if only one person was deserving to go to heaven, who would it be, Narada replied that it would be Shibi and not even himself. Once a brahmin approached Shibi and was asked by him what food he would prefer. The brahmin replied that he wanted the flesh of Shibi’s only son Brihadgarbha. The king cut up his son, cooked the pieces and went to find the brahmin. But he saw the angry brahmin burn down the palace, treasury, armour house, elephant stable etc. The king still invited the brahmin to have his meal. The brahmin sat silently before the food for sometime, and then asked the king to have it and said that only then would he be satisfied. The king started to eat it, when the brahmin showed his true identity. It was Brahma and he brought back Brihadgarbha to life, and disappeared after blessing Shibi, as per Mahabharata Aranyaparva Chapter 168.

Ashtaka was a rajarishi as per Mahabharata Adiparva Chapter 86 verse 5. He gave away his good karma to Yayati, according to Mahabharata Udyogaparva Chapter 122 verse 13-14.