In Vajrayana Buddhism, the Diamond Realm (Skt. वज्रधातु vajradhātu, Jp. 金剛界 kongōkai) is a metaphysical space inhabited by the Five Tathagatas. The Diamond Realm Mandala is based on an esoteric Buddhist sutra called the Vajrasekhara Sutra.
The Diamond Realm is a very popular subject for mandalas, and along with the Womb Realm (garbhakoṣadhātu) Mandala forms the Mandala of the Two Realms. This mandala, along with the Womb Realm, form the core of Japanese esoteric, or Shingon Buddhist, rituals, including the initiation or abhiṣeka ritual. In this ritual, new initiates are blindfolded, and are asked to toss a flower upon a mandala. Where the flower lands helps decide which Buddhist figure the student should devote themselves to.
In traditional Shingon halls, the Diamond Realm Mandala is hung on the west wall symbolizing the final realization of Mahāvairocana Buddha. In this setting, the Womb Realm Mandala is hung on the east wall, symbolizing the young stage of Mahāvairocana Buddha.