First Guru Shiva in the form of Adinath

Being the source of all knowledge of yoga and tantra, he transmitted the secret truths to his student Matsyendranath, who in turn dedicated Gorakshanath to them.

Shiva from the beginning has no form, He is often identified with emptiness or unlimited light (prakasha), he is chit-sakti, unlimited consciousness. Shiva in the form of Adinath is Jyoti-svarupa, the essence of spiritual light. He is a symbol of the unmanifest transcendental reality of "Shunya" or "Shark."

Adinath is revered by all yogis as the perfect Maha Yogesvara, free from all attachments, limitations, unaffected by ignorance, ego, feelings of attachment and disgust. He is forever filled with infinite wisdom, knowledge, power, endless love and mercy. The unborn, immortal, eternal master of all manifested beings and the eternal source of spiritual inspiration for all who seek the truth in this world. He is Ishwara in the Yoga Sutras of Patanjali and Shiva, the Supreme God of all gods, in the Svetasvatara Upanishad. He is the Supreme Spiritual Personality, forever enjoying absolute happiness in his transcendental nature, forever manifesting his own infinite power and beauty in various manifestations, in this magnificent cosmic system. He is the one who constantly shows his mercy to all who sincerely and seriously seek truth, peace, beauty, perfection and self-realization.

It is believed that He manifests himself as the Absolute Truth and universal yoga system, that He is the original author of the agamas (or tantras), which represent the spiritual basis of the yoga system.

A story is related to Adinath, which is somewhat similar to the early Vedic descriptions of the creation of the universe. Adinath emerged from the emptiness of “shunyata”.

Ketaka is the Shakti of Shiva-Adinath, who manifested Brahma from the mouth, Vishnu from the forehead, and from Yoni Shiva. “Keta” means desire; it is the will of Parameshvara Adinath. Legend has it that Adinath decided to test this trinity of gods and turned into a corpse floating on the water. Brahma saw him and pushed him away from himself, Vishnu simply stepped aside, and Shiva took him in his arms, then to cremate. When burned from smoke coming from parts of the body of Adinath, the most famous Nath Yoga appeared. From the bones - Jalandhranath, from the hair - Gorakshanath, from the navel - Matsyendranath, from all parts of the whole body - Chauranginath.

This is what the Siddha Siddhanta Paddhati says:

1.5. The Unnamed (Absolute) is Self-Existing (Parabrahman or Shiva), Beginnerless and Perfect, One, (in which) the (existence) of place and (time) ceases. He is chanted in the Siddha tradition of Siddha Siddhante. The Shakti (Nija-sakti) which is inherent in Him is glorified, following His will (manifesting) as dharma and adharma.
In India, many believe that Adinath was a historical person who lived somewhat earlier than Matsyendranath and Gorakshanath. Adinath is portrayed in human form as a yogi.

The archetype of Adinath in the Vedic tradition can be attributed to Hiranyagarbha or the sacrificial Purusha of the Rigveda. Patanjali mentions him as Purusha-Visheha, and Muslim Naths in Pakistan and Afghanistan consider Adinath Adam, one of the first five prophets, that is, Adinath for them is Adipurusha, the first person. The Jains consider Adinath the first tirthankara.