Wikipedia
Mahasiddha ( Sanskrit: mahāsiddha "great adept; ) is a term for someone who embodies and cultivates the " siddhi of perfection". They are a certain type of yogin/yogini recognized in Vajrayana Buddhism. Mahasiddhas were tantra practitioners or tantrikas who had sufficient empowerments and teachings to act as a guru or tantric master. A siddha is an individual who, through the practice of sādhanā, attains the realization of siddhis, psychic and spiritual abilities and powers. Their historical influence throughout the Indian subcontinent and the Himalayas was vast and they reached mythic proportions as codified in their songs of realization and hagiographies, or namtars, many of which have been preserved in the Tibetan Buddhist canon. The Mahasiddhas are the founders of Vajrayana traditions and lineages such as Dzogchen and Mahamudra.
Robert Thurman explains the symbiotic relationship between Tantric Buddhist communities and the Buddhist universities such as Nalanda which flourished at the same time: