The Diamond Vehicle (རྡོ་རྗེ་ཐེག་པ་) or Vajrayāna is an offshoot of Mahāyāna Buddhism and the last phase of Buddhist development in India before it spread to Bhutan and other parts of the Buddhist Himalayas. This tradition claims to attain the adamantine state of the Buddha through leveraging the adamantine nature of the reality as the adamantine path, and transmute the ordinary body, speech and mind into the adamantine enlightened body, speech and mind (སྐུ་གསུང་ཐུགས་རྡོ་རྗེ་) of the Buddha. Thus, it is called the Adamantine or Diamond Vehicle. It is also known as the result vehicle (འབྲས་བུའི་ཐེག་པ་) for using the resultant state of the enlightenment as the path. However, it is most commonly known as Secret Mantra (གསང་སྔགས་) for its esoteric nature of practice and profuse use of mantra (སྔགས་) spells. Today, it is also known as tantric Buddhism as the tradition is mainly based on Buddhist literatures known as tantras (རྒྱུད་).
Traditional followers of Vajrayāna claim that the esoteric teachings of Vajrayāna were taught by the historical Buddha or some celestial Buddhas but not propagated in the world until it was right time. Tantras such as Kālacakra (དུས་འཁོར་), Cakrasaṃvara (འཁོར་ལོ་སྡོམ་པ་) and Guhyasamaja (གསང་བ་འདུས་པ་), are said to have been delivered by the historical Buddha, often in a non-human celestial form, and then transmitted secretly through a line of vidhyadhara (རིག་འཛིན་) or wisdom holders. Some tantric teachings are said to have been imparted in celestial realms such as Akaniśṭa by non-historical Buddhas such a Vajradhara. However, modern historians argue that Vajrayāna tradition resulted from a process of syncretic development in the first millennium. Building on the philosophical and moral theories of Mahāyāna Buddhistm and the use of mantras, dhāraṇī (གཟུངས་) spells and rituals in both Vedic and Buddhist traditions, the system is said to have developed through a process of innovative amalgamation of Buddhist ideas and non-Buddhist tantric practices in India, particularly that of the Śaiva tradition. Later tantras such as Kālacakra and some Dzogchen tantras are thought to have been written after interaction with the Islamic world and Chinese spirituality respectively.
The Bhutanese and Himalayan tantric tradition classifies the numerous tantras into the action (བྱ་རྒྱུད་), conduct (སྤྱོད་རྒྱུད་), yoga (རྣལ་འབྱོར་རྒྱུད་) and the unsurpassable yoga tantras (རྣམ་འབྱོར་བླ་མེད་རྒྱུད་). The last category is further divided into the father (ཕ་རྒྱུད་), mother (མ་རྒྱུད་) and non-dual tantras (གཉིས་མེད་རྒྱུད་). The Nyingma tradition calls the first three classes of tantras outer tantras (ཕྱི་རྒྱུད་) and the last one, inner tantra (ནང་རྒྱུད་), and divides the inner tantras into mahāyoga, anuyoga and atiyoga traditions.
The Vajrayāna path includes a wide spectrum of practices ranging from austerity and purificatory penance common in outer tantric traditions to uninhibited engagement in sensual pleasures and ruthless aggression in inner tantric schools. Moreover, the Vajrayāna path is strongly associated with the use of numerous expedient methods and techniques, some of which are radical procedures to stimulate enlightenment swiftly and easily. There is pervasive use of mantra spells, mudrā gestures, material enjoyments, artistic expressions, and above all bodily energies and powers in order to stimulate spiritual transformation. Vajrayāna masters adopted many unconventional and rebellious methods of provocation and transcendence to challenge ordinary predispositions and prejudices, and break through the chains of conceptual, cultural and social constructions. It is in the context of such expedient practice that we find Vajrayāna tradition, in contrast to non-tantric Buddhism, elevating the status of human body and women, both of which are seen as being spiritual potent.
As many practices are radical and prone to abuse, the Vajrayāna path is considered very swift and effective but also risky for those who are not ready. Thus, the Vajrayāna teachings were dispensed with restriction, often in secrecy after intiatory rites. The early champions of Vajrayāna tradition such as the Mahasiddhas were often mavericks and laities who carried out the unconventional practices on the fringes of society, and Vajrayāna did not really gain popularity in the mainstream Buddhist centres in India. However, when Buddhism spread in Bhutan and Tibet, the Vajrayāna tradition became the dominant tradition in these countries.
The Vajrayāna system is characterised by the use of the eleven elements of tantra (རྒྱུད་ཀྱི་་དངོས་པོ་བཅུ་གཅིག་) including view, concentation, maṇḍala, accomplishment, conduct, offering, activities, empowerment, samaya precepts, mantra spells and mudrā gestures. Its diverse practices can be classified into two aspects of the development or generation stage (བསྐྱེད་རིམ་) and the dissolution or completion stage (རྫོགས་རིམ་). The former mainly deals with the visualisation and actualisation of divinity to overcome the mundance perception of existence and the latter with dissolution of mundance conceptual and emotional constructions. The diversity of techniques and methods is also reflected in the multiplicity of enlightened divinities with most tantras presenting a distinct Buddha figure. While some Buddhas are peaceful, many others are wrathful and fierce. Many Buddhas have multiple heads and limbs, some have animal heads and many are portrayed in sexual union to symbolize the synergy of wisdom and compassion.
Almost all religious ceremonies, practices, rituals and religious art works in Bhutan today fall within the Vajrayāna tradition. Among the Vajrayāna practices, Bhutanese practitioner often engage in two of the most advanced meditation techniques: the Great Perfection of Dzogchen (རྫོགས་ཆེན་) tradition passed down primarily through the Nyingma school and the Great Seal or Chagchen (ཕྱག་ཆེན་) tradition passed down primarily through the Kagyu school. Students of Vajrayāna in Bhutan also undertake the ngondro preliminary practices before taking up the actual Vajrayāna practices.