From the Super Mario Wiki
Bowser, holding the Star Rod
The Star Rod is a magical rod that can make all wishes come true. According to Russ T., ancient magic created the Star Rod long ago. Bowser steals the Star Rod from Star Haven and captures as well as imprisons the seven Star Spirits at the beginning of Paper Mario. He uses it to grant all his evil wishes. Activating it, he easily defeats Mario by making himself invincible. Mario and his partners must save the seven Star Spirits and bring back the Star Rod.
In the final two battles against Bowser, the Koopa King is aided by Kammy Koopa who uses the Power Platform to increase Bowser's strength on top of the the strength the Star Rod had already given him.
Names in other languages
Sutā no Tsue
||Star Rod (literal translation)
Xīng zhī Zhàng