Siddhas in Hinduism are those who have realized themselves and attained extraordinary powers. Since the time of the Hindu epics, the Mahabharata and the Ramayana, popular portrayals of a group of demigods known as siddhas have figured in the pantheons of South Asian Vedic Sanatani Hindus, Buddhists, and Jainas. In Hindu epics, these beings already figure prominently in the pageant of the upper regions: whenever a hero or exalted self performs some great deed or travels to the atmospheric regions, a host of siddhas (literally perfected beings), accompanied by some other demigods and divine sages, sing his praises and shower him with flowers.
Siddhas were and in some cases they remain, the object of popular sets, identified with mountaintops or the atmospheric region. Amarakosha of the 5th century CE (I.1.11) classes them, together with a number of other beings, as devayonayah, demi gods ‘born from a divine womb’ and therefore not subject to death. Of course, their divine birth too must have been karmaphala (a result of past deeds) of their sublime spiritual efforts in earlier births as human beings.
Gradually, however, the notion arose that the realm of siddhas was one to which humans too could accede, through the intensive practice of Yoga and prolonged penance, and so it was that in the middle ages a growing pool of human siddhas and an expanding body of siddha legend came to be constituted.
Yogaustra commentator Vyasa evokes the possibility of the transformation of humans into semi-divine siddhas in his commentary on Yogasutra of Patanjali (III.51). Later, near about the 6th century CE, a Buddhist group, retrospectively identified as the eight four mahasiddhas or siddhacaryas begins to emerge; a number of these are authors of Caryapadas or Caryagiti, a compendium of mystic poems written in old Bengali, which identify the principal themes of siddha literature: sadhanas (mystic practices) leading to the transformation of the human being into a siddha being possessed of siddhis (supernatural powers).
Hindu siddha groups, lineages, sects or monastic orders appear in later centuries: tantric Siddha Kaula, rasa siddha alchemists, Mahesvara siddhas, Tamil Siddhas and Natha siddhas. Their literature proposed the following working principle – mere humans could, through their intense yogic, alchemical or erotico-mystical and deep devotional worship practices, climb the ladder of being and accede to the ranks of the semi-divine siddhas. The Siddha sects flowed into the historical Siddhar orders. It worked both ways – besides the advent of some nobles selves due to pious sadhana and prolonged penance on the one hand, on the other even ordinary human practitioners, engaging in a number of interrelated mystic techniques, began to claim themselves the title or status of Siddha
A rich siddha hagiography quickly arose, often conflating these human champions of immortality with the semi-divine figures whose elite throng they were said to have joined. Kathasaritsagara and other medieval works of adventure and fantasy literature abound in accounts of such transformation. Hagiography became rationalized into guru-shishya (preceptor-disciple) lineages, making Gods and demigods the founders of human religious order or lineages. A wide array of Sanskrit and vernacular medieval sources from the tantric, alchemical, and hatha yoga traditions offer intersecting lineage lists of siddhas, which blur the lines between God, demigod, and human, as well as between Sanatana Vedic Hindu, Buddhist and Jainas.
The 14th-15th century CE, Shankaravijaya of Anandagiri, which devotes its 49th chapter to a description of siddhas, presents a siddha charter of sorts, which emphasizes the most salient features of siddha practice. Having gained possession of special herbs and mantras at Srisailam and other lofty sites where divine beings make themselves visible, Satyanatha and others became Siddhas, persons who had realized their goal and long life. We are living according to their precepts, through our special knowledge and special expertise in gaining mastery over each of the five elements, and by virtue of drinking poisons, drinking mercury, and drinking specially prepared oils and by means of special forms of yogic practice. We ensure the removal of accidental or untimely death by means of special acts of sorcery, through special shaktis, yakshis and mohinis by means of iron making, copper making, silver making, gold making, etc., and by means of the various sorts of metallurgical expertise and through the special use of black mercuric oxide, roots, and mantras.
The allusion to the consumption and use of mercury refers to the alchemy of the rasa siddhas, who flourished between the 10th and 14th century CE., who combined their alchemy with hatha yoga and tantric practice. This group has left behind a significant body of literature in Sanskrit, of which the most important works are Rasahridaya Tantra, Rasarnava, Kakachandesvarimata, Rasaratnakara and Rasendra Mangala, by different authors.
The mention of Satyanatha in the above passage is a reference to the most successful and long lived of siddha groups: this is the Natha Sampradaya, a Shaiva religious order founded by Gorakhnath (Gorakshanatha) in northwestern India in the late 9th century CE. A Natha Siddha legend spuriously identifies Gorakhnath as the disciple of Matsyendranath, the 9th century CE. author of Kaulajnananirnaya and member of the tantric Siddha Kaula. There are nonetheless good reasons for Gorakshanatha's close association with Matsyendranath. Kaulajnananirnaya is the earliest Hindu source to give a quite comprehensive account of six-chakra hatha yoga, and it is precisely on this subject that Gorakhnath is recognized in India as one of the greatest synthesizers and innovators.
Gorakhnath was a prolific author, whose many Sanskrit works on the techniques of hatha yoga remain authoritative. The most important of these are Goraksha Sataka, Siddha Siddhanta Paddhati and Amanaska Yoga. An alchemical work, Bhutiprakarana, is also attributed to him as a tantric work. In addition, Gorakhnath left behind and extensive body of old Rajasthani poetry on various facets of yogic practice and experience. The banis of Gorakhnath were an important source of inspiration for a number of later Natha siddha poets, including Cauranginatha, Carpatinatha, and Bhartrhari, and Gopicanda. Among other earlier and later natha siddhas, some are Adinatha, Kalanatha, Karalanatha, Vikaralanatha, Kalabhairavanatha, Batukanatha, Bhutanatha, Viranatha, Niranjannatha, Udyanatha, Suryanatha, Kurmanatha, Nivruttinath, Jnaneshwaranatha and so on.
The rich hagiography of these and other Natha siddhas are found in such sources as the 14th century CE Marathi Yogisampradayaviskrti (attributed to Jnaneshwara); the 15th century CE Telugu Navanathacaritra of Gaurana; the 16th century CE Bengali Goraksha Vijaya of Sheikh Fayzulla; and on the lips of singers of their song cycles in present-day India.
Source – Encyclopedia of Hinduism Volume IX – IHRF – Rupa Co – page 427 - 429