Wikipedia
Saṁdīpanī Muni was the guru of Bhagavan Śrī Kṛṣṇa. Saṁdīpanī was a ṛṣi/muni/saint of Ujjain.
Saṁdīpanī means, "Possessor of complete (sam-) illuminating (-dīpana-)." The Saṅgīta-Sāra-Saṅgraha relates the name as a particular musical śruti (text).
The Saṁdīpanī Muni āśrama is located 2 km outside Ujjain, in the Indian state of Madhya Pradesh. The area near the āśrama, known as Aṅkapāta, is popularly believed to have been the place used by Bhagavan Śrī Kṛṣṇa for washing his writing tablets. A locally-sourced narrative relates that the numerals 1 to 100 found inscribed on a stone were originally engraved by Saṁdīpanī.
Near the āśrama is the Gomti Kund, a stepped water tank. Legend has it that this is where Kṛṣṇa summoned all the holy waters from various centres so that his elderly Guru, Saṁdīpanī Muni would not have to travel other holy places.
The Bhāgavata Purāṇa relates the following story regarding Saṁdīpanī Muni: While staying as students at the residence of Saṁdīpanī Muni, the two brothers—Śrī Kṛṣṇa and Balarāma— and their friend, Sudama (Su-dāma), 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 dakṣiṇa (his fee for providing instruction) of his own choosing. Saṁdīpanī asked for the restoration of his child, who had disappeared in the ocean at Prabhāsa (on the Western Coast Gujarat state of India, close to Somnath temple). The two brothers traveled to Prabhāsa and found that the son had been snatched away by a being named Śaṅkhāsura (literally, "conch demon"; In the Mahābhārata, Śaṅkha is mentioned as one of Kubera's treasures, as well as the being presiding over it).
According to the Bhāgavata-Purāṇa , Śaṅkhāsura inhabited a beautiful conch named, panchajanya ( IAST: pāñca-jana; literally, "Five-People," mentioned in the Aitireya Brāhmaṇa as the name of a group of inimical tribes, but which may also be connected with the constellation, Boötes, which resembles a conch shell), lived under the waters in the shape of a conch). Not finding the son within the conch, Sri Krishna and Balarama took the conch and went to Yama (who is also likely associated with Boötes), and blew the conch. Yama worshiped both of them saying, ‘O Viṣṇu (One Who Pervades the Universe), disguised as a human being by way of līlā Lila (Hinduism) (play), what can we do for you both?’ Kṛṣṇa replied: ‘Impelled by My command, O great ruler, fetch my guru's son, who was brought here as a result of his own karma.’ Being brought back to life, they returned Saṁdīpanī's son. It was thus in the process of rescuing his guru's disciple from the clutches of Death-personified (Yama) that Śrī Kṛṣṇa acquired his famous conch, Pañca-Jana, from Śaṅkhāsura.