Sandipani

Sandipani (सान्दीपनि), sometimes rendered Sāndīpana, is the guru of Krishna and Balarama in Hinduism. He is regarded to have educated them regarding all the Vedas, the art of drawing, astronomy, gandharva veda, medicine, training elephants and horses, and archery.

Legend
The Bhagavata Purana relates the following story regarding Sandipani: While staying as students at the residence of Sandipani, the brothers Krishna and Balarama, and their friend, Sudama, mastered every single lesson, although only having been instructed in each once. Upon the rapid completion of their studies, they persuaded their teacher to ask for the preceptor’s dakṣiṇā (a type of honorarium to one's guru) of his own choosing. Sandipani asked for the restoration of his child, who had disappeared in the ocean at Prabhasa. The two brothers travelled to Prabhasa and found that the son had been snatched away by a being named Śaṅkhāsura (lit. 'conch demon'). Krishna rescued his son and they returned him to their preceptor.

In the Harivamsha Purana, Krishna learnt that Sandipani's son has been swallowed by an asura called Panchajana, and had perished. Krishna and his brother travelled to Yamaloka to persuade Yama to restore his preceptor's son back to life, and succeeded:

"He then brought from Yama’s abode his preceptor’s son lost for a long time. By the power of Krishna of peerless energy Sandipani’s son, dead long ago, returned in his bodily form. Beholding this highly wonderful feat which cannot be thought of or performed by any all the creatures were filled with surprise. Taking his preceptor’s son, Pancajanya and diverse precious jewels, Madhava, the Lord of the universe, returned."