Saweet

Archer came in contact with Rho Aias: The Seven Rings that Cover the Fiery Heavens ( 熾天覆う七つの円環ロー・アイアス,  Shiten Ōu Nanatsu no EnkanRō Aiasu ?, localized as 7 Rings of Heaven) at some point in his life, and added it to his library of weapons and defensive armaments stored in Unlimited Blade Works.

In its greatest form, it is said to be seven flower petals made from light. It takes the form of a seven layered Bounded Field in the shape of an iridescent flower in reference to the rocket larkspur (Consolida ajacis) flower that bloomed from Aias' blood as he died, with seven petals representing the seven layers of oxhide. Each flower petal layer has the defensive power of a fortress wall, and Archer cannot personally recall any thrown lance that would be capable of penetrating even the first layer. It is necessary for its user to provide constant magical energy to form the shield, and if it is damaged, they will feel pain and receive damage in accordance.

He uses it as his main defense, as it is the one in which he has the most proficiency with, and Shirou Emiya cites it as Archer's greatest protection. He uses it to defend against Cú Chulainn's Gáe Bolg, which manages to pierce up to the seventh layer before finally stopping. This action takes much of his energy, as it takes three times the amount of magical energy as a weapon in order to project it, and still leaves his arm and body damaged afterward. He can also project it from a distance, allowing him to shield Shirou from Gilgamesh's Enuma Elish without either of them realizing it.[3] If it is used by Shirou after having access to Archer's knowledge, he is only capable of generating four petals in Heaven's Feel and Fate/hollow ataraxia. It is still powerful enough to help Bellerophon overpower Excalibur Morgan and temporarily deflect a charged shot of Hrunting. Shirou is able to project the full version during Unlimited Blade Works after "pulling it from the hill" of the Reality Marble in order to protect himself from Gilgamesh's onslaught of swords.[4]

In the alternate universe of Fate/kaleid liner PRISMA☆ILLYA, in the alternate dimension of the Ainsworth, Shirou did get Rho Aias somehow. He used it to protect Rin Tohsaka, Luvia Edelfelt and Bazett Fraga McRemitz from a massive ranged attack by the Grail mud-born 'heroes' who were under control of Julian Ainsworth. Kuro can also actualize Rho Aias, albeit with only four petals.[5