
Guhyagarbha tantra

The Guhyagarbha Tantra (Skt.; Tib. རྒྱུད་གསང་བ་སྙིང་པོ་, "Gyü Sangwé Nyingpo"; Wyl. "rgyud gsang ba'i snying po", "The Tantra of the Secret Essence" or the "Secret Womb Tantra") is the most important Buddhist tantra of the Mahayoga class and the primary tantric text studied in the Nyingma tradition. It is the main Nyingma source for understanding empowerment, samaya, mantras, mandalas and other Vajrayana topics, and has influenced the Dzogchen tradition. The Nyingma scholar Longchenpa sees it as "the highest summit of all vehicles, the source of all verbal transmissions, the great great shortcut of the vehicle of all Buddhas of the three times, the most secret."
Overview.
Its full title is the Glorious Web of Magical Illusion, The Secret Essence Definitive Nature Just As It Is (Tib. "Pal Gyutrül Drawa Sangwé Nyingpo De Kona Nyid Ngé Pa", Wyl. "dpal sgyu 'phrul drva ba gsang ba'i snying po de kho na nyid nges pa"). It is also often called The Tantra of the Web of Magical Illusion (Tib. "Gyutrül Drawa Gyü", Wyl. "sgyu 'phrul drva ba rgyud"). 
The Nyingma school holds that Garab Dorje received the empowerment and transmission of the Mahayoga teachings of the Guhyagarbha from the indian Mahasiddha Kukuraja.
The "Secret Womb" actually refers to a collection of nineteen texts in the Tibetan canon. The main works include a short version in twenty-two chapters, an intermediate version (forty-six chapters) and a long version (eighty two chapters). These texts describe a mandala of forty-two peaceful deities and fifty-eight fierce deities. According to John Powers, its central teaching "holds that all things manifest spontaneously ("thams cad rang snang"), and mind and primordial wisdom also manifest spontaneously ("sems dang ye shes rang snang")." The "Secret Womb" texts discuss numerous tantric Buddhist topics, such as the creation of mandalas, the practice of controlling the winds and drops within energy channels, the purification of the five aggregates, and the qualities and activities of the Buddhas. 
Tibetan interpreters generally see the "Secret Womb Tantra" as a guide to the Buddhist practice of Mahayoga, also termed deity yoga, a tantric method of realizing the true nature of reality and attaining Buddhahood. This method mainly relies on the imagination to create a Buddha image in one's mind during a tantric ritual and then to merge this Buddha with oneself. 
According to Gyurme Dorje: 
"The iconography and symbolism of the hundred peaceful and wrathful deities presented in the Guhyagarbha Tantra subsequently gave rise to a whole genre of literature in Tibet known as the Cycles of the Peaceful and Wrathful Deities ("zhi-khro")." 
The "Tibetan Book of the Dead" ("Bardo Thodol") is one such "zhi-khro" text.