Yuukai (1345-1416) also played an important role in purging what was known as the Tachikawa school. ... The Tachikawa teachings were extirpated by orthodox Shingon: Yuukai burned all the writings of the Tachikawa school at his temple on Kouya-san, saving only a list of the texts destroyed.
(Taikou Yamasaki: Shingon - Japanese Esoteric Buddhism, p. 44)
The Tachikawa school appears to have indulged in the sexual rites practiced by the somewhat similar Shaktist sects of Tibet. In 1335, as the result of a memorial submitted by the Mount Kouya mongs against the Tachikawa school, its leader was exiled and books expounding its principles were ordered to be burned. Traces of its doctrines still survive, however, in existing Buddhist sects.
(Yoshiko Kurata Dykstra: Sources of Japanese Tradition, p. 180)
This Tachikawa school later spread to the province of Etchuu. In successive generations, two teachers, Kakumei and kakuin, lived on Mount Kouya [and taught Tachikawa doctrine there]. At this time, many secret manuals and texts of this heretical school were in circulation, often called "oral transmission of the secrets of esoteric doctrine." To this day, there are ignorant people who study such works and believe them to possess the loftiest thoughts. In truth, they are neither exoteric nor esoteric but merely so many stones wrapped in jade. ... Many people studied these teachings, but they did not meet with devine favor, and for the most part, both the teachings and the men have perished. A few are left, but i do not know how many.
(Sources of Japanese Tradition, quotes from Yuukai's Houkyoushou: "TD 77, no. 2456:847-849", quote on p. 189)
It is in Shingon and Tendai Buddhism that we find for the first time two movements that have been commonly labeled "heresies" (jakyou). For all its radical criticism of established Buddhism, even the Nichiren school was not disqualified by this label, and it remained a powerful trend within Buddhism. The Shingon and Tendai traditions, however, tried for centuries to assert a rather problematic distinction between "esoteric Buddhism", or "pure esotericism," and Tantrism (or "mixed esotericism"), that is, a form of Tantrism unexpurgated of its darker magical (and in particular sexual) elements.
The Tachikawa branch is said to have emerged during the Kamakura period, with the teachings of Ninkan (d.u.) and Monkan (1281-1357). It advocated sexual union as the fusion of the two mandalas and as the technique leading to the apotheosis called sokushin joubutsu ("becoming a buddha in this very body"). Certain aspects of the Vajrayana, which were considered if not entirely orthodox at least acceptable in Tibetan Buddhism, came to provoke strong reactions on the part of conventional Japanese Buddhists. Consequently, the Tachikawa movement was forbidden during the Muromachi period. Despite its formal disappearance, however, its influence lingered and was felt in many places, in the imperial house as well as in Shingon and Zen monasteries.
(Bernard Faure: The Red Thread, p. 126-127)
"While teachers of the middle way, mind only, transcendent wisdom, mantra, and other schools may have their own assertions, the fulfillment of those intentions is the same. There is not a single thing that is not contained within mind."
(Gampopa to Düsum Khyenpa, in "The First Karmapa", KTD Pub, p254)
“If you recognize the world of appearance and existence as the mind, realize the mind itself as empty, and have no grasping at the superiority of your realizations — this is the ultimate view."
(Chegom Dzongpa, in "The Book of Kadam", Wisdom Pub, p609)