Sri Singha
Sri Singha was the teacher of Padmasambhava, Vimalamitra, and Vairotsana. He was a principal student and dharma-son of Mañjuśrīmitra in the Dzogchen lineage, and is credited by the Nyingma school with introducing Dzogchen to Tibet.
Life
Sri Singha is the son of King 'Accomplisher' and his wife queen Nantakā.A.W. Barber notes that Sri Simha took the Atiyoga lineage to Andhra, in South India. He made his residence at Dhanyakataka.
Thus, it would appear that Sri Simha took the Atiyoga line to Andhra and made his residence at the famous Dhanyakataka along the Krishna River. From here it was transmitted to teachers who then took the line to Tibet and China.
Achievements
Śrī Siṃha brought the Secret Mantra teachings from beneath the Vajra Throne in Bodhgaya to the 'Tree of Enlightenment' in China, where he concealed them in a pillar of the 'Auspicious Ten Thousand Gates Temple'.Śrī Siṃha conferred the Eighteen Dzogchen Tantras upon Padmasambhava. The eighteen are The Penetrating Sound Tantra, to which was appended the Seventeen Tantras of Innermost Luminosity.
Kunsang, in rendering the instructions of Sri Singha to Padmasambhava, writes that:
In general, all phenomena belonging to samsara and nirvana are, from the very beginning, spontaneously perfected as the essence of awakened mind. However, because of failing to realize and not knowing this to be just how it is, sentient beings circle among the three realms and continue to wander among the six abodes. In order to guide them, it is generally said that an inconceivable number of doorways to the Dharma belonging to the various vehicles have been taught, but these can all be contained within development, completion and the Great Perfection.
Kunsang, in rendering the continued instructions of Sri Singha to Padmasambhava, outlines:
...there are the three sections of the philosophical vehicles belonging to nirmanakaya, the three sections of Kriya and Yoga belonging to sam-bhogakaya, and the three aspects of development, completion and the Great Perfection belonging to dharmakaya. Among these, I shall explain to you the Ati Yoga of Instructions, distilling its quintessence, which is the Innermost Vajra Essence of the Unexcelled Fruition. So listen, Padmasambhava.
'Philosophical vehicles' in the abovementioned quotation denotes Theravada and Mahayana as different from Vajrayana.