Śubhakarasiṃha 善無畏 (637–735) is noteworthy as one of the early tantric adepts to visit China and introduce a number of texts and practices in addition to providing initiations (abhiṣeka). He was a contemporary of Vajrabodhi 金剛智 (671–741) and the young Amoghavajra 不空 (705–774). He is most known for his translation of the Mahāvairocana-sūtra 大日經 (T 848), which was done with the assistance of the Chinese astronomer monk Yixing 一行 (683–727), though he was also active at court and was responsible for other texts. He was born into the royal family of Orissa, but he renounced the throne when his brothers initiated a violent struggle over the succession. In light of his royal background and training at Nālandā, he was likely the most educated Indian monk to live in China in the eighth century (he arrived in 716). He was instrumental in the transmission of early Tantra into China. He might also have been disciple to the human author of the Mahāvairocana-sūtra. There is an interesting document from 834 detailing the history of the maṇḍala lineages which quotes Śubhakarasiṃha as follows.
《兩部大法相承師資付法記》卷2：「三藏善無畏云：此法從毘盧遮那佛。付囑金剛手菩薩。金剛手菩薩，經數百年，傳付中印度那爛陀寺達磨掬多阿闍梨。達磨掬多阿闍梨次付中印度國三藏釋迦種善無畏。」(CBETA, T51, no. 2081, p. 786, b5-9)
Tripiṭaka Śubhakarasiṃha said, “This Dharma is from Vairocana Buddha. It was entrusted to Vajrapāṇi Bodhisattva. After hundreds of years Vajrapāṇi Bodhisattva entrusted it to ācārya Dharmagupta from Nālandā monastery in Central India. The ācārya Dharmagupta then entrusted it to Tripiṭaka Śubhakarasiṃha of the Śākya clan from Central India.”
This indicates that the early tantric literature was put down in writing in major centers of Buddhist learning such as Nālandā in Magadha. In the case of the Mahāvairocana-sūtra, Dharmagupta is said to have received it from Vajrapāṇi, which seems to mean he was responsible for penning it. The history of the lineage in India before its introduction to China, and thereafter to Japan first via Kūkai 空海 (774–835), was surprisingly short.
The lineage was also transmitted to Tibet and is still maintained, though not necessarily practiced, at least in the Sakya school.
I am unsure if there are any details available on Dharmagupta. We might imagine he flourished in the mid-seventh century at Nālandā. He might have been a contemporary resident of Nālandā when Xuanzang 玄奘 (602–664) stayed (he arrived in India in 633 and returned to China in 645). As far as I know Xuanzang never mentions the existence of such a lineage even though he was a master of dhāraṇī. One gets the sense that ‘Tantra’ as we would recognize it with abhiṣeka and maṇḍala emerged relatively quickly, though many of its elements existed beforehand in various forms. Dharmagupta might have been one of the early innovators and initiators of Buddhist Tantra. It is furthermore quite instructive that he was from Nālandā as it narrows down the general location we ought to search for in seeking the origins of Buddhist Tantra. It also raises questions concerning the social and political climate which existed at the time in Magadha to foster such developments. It seems it was a time of ongoing conflicts, especially after the collapse of the Harṣa-vardhana dynasty in the mid-seventh century.
This brings to mind the point that one can learn a lot about ancient India through reading Chinese sources. Indology without Classical Chinese is subject to a severe handicap. For example, in my research I’ve uncovered a number of things about Indian calendars and astrology that are never mentioned in Indological resources such as modern monographs and encyclopedias that treat such subjects. Although the scholars involved certainly all know Sanskrit quite well, the reality is that a great amount of Indian literature is preserved in Chinese, to say nothing of all the accounts of India as written by East Asian monks such as Faxian, Xuanzang, Yijing and Hyechao. The latter have all been translated into English, but there are still other accounts of India that are preserved in secular Chinese histories which have brief accounts of foreign countries. Just as an example, I compiled all the accounts I could find of Nepal in Chinese sources and translated them (see here). If you dig around Chinese sources – secular and Buddhist – which are increasingly digitized, there is much data one could collect about ancient India and even Persia (see here).
What this really points to is the artificial barriers between fields like Indology and Sinology. They really ought not to be treated so separately as they complement one another. I suppose this is the advantage that Japanese Indologists have had over Western Indologists – they generally read Classical Chinese and can refer to those sources. This was especially the case in past generations when Japanese scholars attained basic literacy in Classical Chinese before entering university, much the same as Latin was expected of European students. This is not so much the case any longer unfortunately.