The sons of Brahma, who had mastered the Vedas and Upanishands at a very young age, the four Kumaras Sanaka, Sanandana, Sanatana and Sanatkumara, came to have Darsana of Lord Siva and prostrated before him. They entreated Lord Shiva to teach them the way to remove the veil of Maya or illusion.
Lord Siva, hearing this appeal made by the sages, assumed the form of Dakshinamurti. He attuned them to the brain wave frequency of Turiya and transferred the requisite knowledge to them by telepathy in total silence.
Lord Shiva Himself came down in the form of Dakshinamurthy and sat under a banyan tree to give enlightenment to the 4 sons of Brahma.
Dakshinamurthy, in the Hinduism is regarded as the first guru -- and the destroyer of ignorance, represented by the demon Apasmarapurusha, (the deluded self) being crushed under the divine foot.