Rasengan

is a powerful A-rank technique invented by the Fourth Hokage, Minato Namikaze, which he created by observing the Tailed Beast Ball. Minato spent three years developing the Rasengan; his plan was to take the shape transformation of one's chakra to the "highest possible point" and then combine it with his chakra's nature affinity, creating a technique that did not need the aid of time-consuming hand seals to control. He died before he could accomplish this, but on shape transformation alone, the Rasengan is still a very useful, powerful, versatile, and difficult technique to learn and master