Eternity Cudgel (定海棒), also called the Ruyi Cudgel (如意棒), As you wish Cudgel, Extendable Staff, Extendable Rod is a weapon that Bai Yuechu uses. It is a staff that can extend to very long lengths.
With it Bai can perform various Rod Arts (棍术), Spirit Arts / God Arts (神通), and other Ruyi Cudgel Arts (如意棒法).
According to Wang Fugui Yeye, the longest the rod can stretch is 40.06 meters, but once Bai Yuechu far surpasses this. Madame Quan senses the mist of Aolai and scent of Huaguo. The cudgel is something all yaoguai worship.[1]
But in the past the rod had broken a few times before: once by Tushan Honghong under an illusion in Season 3 and another time by the Lava Dragon who had stolen spirit power from the single-eyed god.[1]
The rod is based on the Ruyi Jingu Bang that Sun Wukong uses in the classic epic novel Journey to the West.