Ugetsu

Ugetsu (雨月, "unable to see the moon because of rain"): Being the strongest technique that can be learned by a Moon-type Zanpakutō, Ugetsu is a technique that is "within it's own tier." Sannotō Kawahiru noted that the technique is rather similar in principle to the Sōzōshin's Sinsaeng Jeongsu. It is formed by firing a massive Getsuga Tenshō into the skies, which halts right above the area of it's initiator's spiritual energy range. At that point, all sources of reishi are collected by the Getsuga technique, no matter the size or density of the spiritual energy. As more gathers, the size of the Getsuga in the skies grows into a large black moon which blocks out the sun (or in Raian Getsueikirite's case, a silver moon). Any beings that are made of reishi that come into contact with the technique are either sucked in, effectively strengthening the technique overall, or completely decimates said target. It is unknown exactly how to stop the technique, but Sannotō notes that only by killing the user of the technique, can stop Ugetsu. However, Seireitou retorts that if one can stop the Getsuga Tenshō from collecting enough reishi, it is theoretically possible to stop it before it gets too powerful.

Behind the Scenes
The idea of Ugetsu actually came from Chibaku Tensei, a technique used by the Sage of the Six Paths in Naruto to form the moon. The name Ugetsu came from a famous japanese play named Ugetsu Monogatari (雨月物語, literally "Tales of the Moon and Rain").