Kongocho-kyo (Diamond Peak Sutra) is a general term for assorted 'Kongocho-kyo' texts that were compiled one after another by a group called "Shoe Kongocho-kyo" (sarvatathāgatatattvasaṃgrahaṃ nāma mahāyānasūtraṃ, or "Mahayana Sutra Named as a Collection of All the Truth of Nyorai," abbreviated as "Shinjitsu Sho-gyo"). The origin of the name 'Kongocho' (vajraśekara) is said to have come from the "Kongocho Tantra" (vajraśekharatantra).
In Japan, 'Kongocho-kyo' usually refers to "Shoe Kongocho-kyo."
"Shoe Kongocho-kyo" is a sutra (or sacred book) used as a reference for Diamond World Mandala, and within the Shingon sect and Tendai sect, it is said to clearly explain the principles of 'Sokushin-Jobutsu' (Attaining Buddhahood with the Present Body) in Esoteric Buddhism.
Within the Shingon sect (Tomitsu (eastern esotericism)) especially, it is regarded as the Konpon kyoten (the most important sutra), with the two sutras of Esoteric Buddhism, "Dainichi-kyo" and 'Dainichi-kyo' (Mahavairocana Sutra) together being called 'Ryobu no Taikyo.'
The "Rishu-kyo" (Principle of Wisdom Sutra) (Adhyardhaśatikā prajñāpāramitā, "Hyaku-goju Ju Hannya" (literally, 150 gatha (poetic verse of a scripture) of wisdom), which is chanted in the Shingon sect, is a simplified sutra of "Rishuko-kyo" and one of the texts of the 'Kongocho-kyo.'
Kukai (a Japanese monk, scholar and poet who lived from 774 - 835) became a disciple of Keika Osho (746 - 805) of the Qinglongsi Temple at Changan of Tang. There he learned the Denpo Kanjo (the ceremony to invest someone with Ajari (rank of master)) of Esoteric Buddhism and learned the principles and true way to practice "Shoe Kongocho-kyo" (passed down from Dainichi Nyorai to Kongosatta, Ryumo, Ryuchi, Kongochi, Fuku, Keika and then Kukai). In 806 Kukai introduced the system of practicing "Shoe Kongocho-kyo" to Japan for the first time.
There is a legend which tells that 'Kongocho-kyo' was gained by perceiving Ryumo in a steel tower in Southern Tenjiku (India). This sutra is a general term for most sutras that involve counting up to one hundred thousand ju (unit for counting poems) and were preached by Dainichi Nyorai at 18 different places on separate occasions and are not a single sutra.
Sutras translated into Chinese
As for sutras which were translated from Sanskrit into Chinese, there are 4 volumes of "Kongocho Yuga Churyakushutsu Nenju-kyo" (Rykushutsu Nenju-kyo) by Kongochi (ajrabodhi) Sanzo (tripitaka) (around 670 - 741), 3 volumes of "Kongocho Issai Nyorai Shinjitsu Shodaijo Gensho Zanmai Daikyoo-kyo" (Kongocho Daikyoo-kyo) (Compendium of Truth of All the Tathagatas), by Fuku (Amoghavajra) Sanzo (705 - 774), and 30 volumes of "Issai Nyorai (concentration of Tathagatas) Shinjitu Sho-gyo Sutra (Tattva-samgraha)" by Sego.
The original text in Sanskrit, along with the translation into Tibetan presently exist as well, and translations by Sego correspond to both. It is believed that from the middle to the end of the seventh century, the basic form of sutras was established in Southern India, and gradually changed to its complete form, as seen in translations by Sego.
The main part of it is about Dainichi Nyorai telling Issaigijoju Bosatsu (Shakyamuni) about his enlightenment against the question asked to him, and the way to practice in order to earn that enlightenment. Diamond World Mandala specifically shows the content of that enlightenment, and the main part of the explanation of the actual practice is the goso jojin kan (meditation procedure to be a Buddha through five phases). Goso jojin kan is an original practice to endow devotees with discovering the wisdom of Nyorai: This practice involved having the impure hearts of disciples attain the knowledge of Nyorai (Tathagata; perfected one) through yoga methods of contemplation; as they realized the pure figure of Nyorai, they would realize the prajna (insight leading to enlightenment), that it could become nothing else and they could become one with Nyorai.
Commentaries by three great scholars of Yuga Esoteric Buddhism in the 8th century, Buddhaguhya, Anandagarbha, and Sakyamitra, remain as translations in Tibetan.
Listed below are four commentaries which still exist today that attached to the Tibetan translation of "Kongocho-kyo" (also known as Shinjitsusho-gyo). "Tantra Ginyu" (Tantra with Buddhaguhya's Commentary): Compiled by Buddhaguhya. "Comments Against the Integration of the Truth, Solemnity of Koala:" Compiled by Sakyamitra. "Comments and Truth of Light Against Tantra, Which is Named Agenda of Mahayana, the Integration of the Truth of Nyorai:" Compiled by Anandagrbha. "Ship Entering the Sea of Yuga Tantra:" Compiled by Bu ston rin chen grub.
Also, as commentary against "Tantra Ginyu" by Buddhaguhya, there is "Commentary of Tantra Ginyu" by Padmavajra.