Senkei

is a Bankai technique of Byakuya Kuchiki's Zanpakutō, Senbonzakura Kageyoshi.

Description
The true form of Senbonzakura is created by his blood, which glows and turns into spiritual energy, mixing with the tiny blades of his Bankai. This unites the scattered petals into complete glowing swords, numbering in the thousands, which form four rows. These rows, floating just above each other, circle around Byakuya and his opponent in a continuously moving, column-like array. It abandons defense in favor of a concentrated offense. Though visually menacing, this form is not meant for attack from all sides; rather, it concentrates his power into fewer blades, thus increasing his attack power. While Byakuya can mentally control these swords as he does his regular blades, he commonly calls them to his hands for actual melee combat. When in his hand, they, losing their glow, take the form of his sealed Zanpakutō. While they seem solid in this state, the swords can just as easily dissipate back into the multitude of tiny blades which they are made up of. It seems to take fewer blows from a Senkei blade to gravely injure an opponent than the multiple cuts needed from the unfocused Bankai. Senkei cages himself in with his enemy, allowing neither combatant to move very far from each other, though the area is still large enough for maneuverability. Senkei's effectiveness lies in it allowing for use of all the swords at the same attack level, making it highly versatile as a way to effectively defeat an enemy unaware of the danger of the situation. It is extremely rare for him to use this ability, doing so only when he has sworn to cut down the opponent with his very own hands. Ichigo is the second being to ever witness the ability.