Vajra (ヴァジュラ, Vajura?) is a weapon appearing in the Vedic Mythology of ancient India. It was a weapon crafted by Tvastar, the maker of divine instruments, using the bones of the sage Dadhichi. It became the symbol of Divinity of Indra, the God of Lightning, and the Indestructible Diamond Pounder (金剛杵, Kongōsho?) weapon carried by the gods of Buddhism. It was held in one interpretation to be a manifestation of Indra’s lightning, which is supported by its depictions as a golden projectile and its appearance unbefitting of a conventional weapon. Vajra was initially known by its proper name of Vjaya when Indra was first introduced to India as a phenomenon that spread from the Aryans. Along with Indra, whose name changed to Taishakuten, his divine symbol also had its name changed and became known as Vajra after the age of Buddhism as India's main religion began.
Kama wields the Vajra Spear in her Second and Third Ascensions. In Hindu Mythology, Vajra is a weapon of Indra, but it is said that Kama got in the way of Shiva's meditation on the orders of Indra. It is probably an indirect criticism of that.
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« If that’s the case, allow me to show you!"
Śakra's Vajra (釈提桓因・金剛杵, Shakudai Kan'in - Kongōsho?) is the Noble Phantasm of Minamoto no Raikou in her swimsuit. The Vajra of Gozu Tennou, also known as the Eastern Deity, Śakra (Indra). In other words, the Ultimate Weapon of the gods made of holy sacrum, "Vajra", descends from the skies, destroying all kinds of enemies―――Because of this Noble Phantasm, this time's Raikou was bound to be under the Lancer class. Its shape is a Dokkosho that represents the single Tathātā of all things in creation as raised in the Vajrayana.
Arjuna: "That Noble Phantasm is father's..."
Rama: "That Noble Phantasm is..."
 Fate/complete material II: Character material - Weapons: Vajra, p.135
A weapon appearing in the Vedic mythology of ancient India. Owned by Gilgamesh. Originally the bones of the sage Dadhichi, and crafted into a weapon by the maker of divine instruments, Tvastar. In the decisive battle against the Asura Vritra, the thunder god Indra used this weapon to slay him.
One interpretation holds that Vajra is a manifestation of Indra’s lightning, supported by the fact that it is often depicted as a golden projectile. Plus, it just doesn’t look very useful as a conventional weapon.
When the myths were absorbed into Buddhism, Indra was replaced with Śakra, and Vajra’s name was also changed accordingly.
ギルガメッシュが所蔵する、古代インドのヴェーダ神話に登場する武器。ダディーチャ聖仙の骨を元に、工芸 の神トヴァシュトリが作り上げたとされている。雷神インドラは悪竜ヴリトラとの決戦時にこの武器を使用し、 悪竜を倒したと伝えられる。
この武器は雷神インドラが攻撃に使った雷を具現化したものだという解釈もあり、金色の投擲武器として描か れることが多い。またこうした理由により，実際の武器としては、非実用的なデザインとなってい る。
 Fate/stay night - Weapon Menu: Vajra [T]
One of the divine symbols of Indra, God of lightning, from ancient Egyptian mythology.
Indra was originally a phenomenon introduced by the Aryans, and Vajra was officially called Vijya.
But when he was renamed to Taishakuten as the age turned to Buddhism, his divine symbol also changed its name.
It is a single-shot Noble Phantasm, and its damage is about B+.
It is a simple weapon that does fixed damage regardless of the possessor's magical energy. Also, Vajra is a weapon that Buddhist Gods possess.
- Fate/Grand Order material V p.319
- Fate/Grand Order Profile of Minamoto no Yorimitsu (Swimsuit), translated by roadromancer at Reddit.