What item does Bowser steal to become invincible in Paper Mario?

Bowser steals the Star Rod to become invincible in Paper Mario.

The Star Rod is a powerful artifact kept in Star Haven, the home of the seven Star Spirits. During the opening crisis, Bowser invades Star Haven, takes the rod, and uses it to defeat Mario and make himself invulnerable. He then imprisons the Star Spirits so they cannot counter its power.

Mario’s adventure centers on rescuing the seven spirits from Bowser’s minions. Once freed, they can combine their strength to neutralize the Star Rod’s effects, making it possible for Mario to challenge Bowser in the final battle. The rod therefore drives both the game’s opening defeat and its overall quest structure.

Players sometimes confuse it with the Crystal Stars from The Thousand-Year Door or the Sticker Comet from Sticker Star. Those belong to later Paper Mario stories; the original game’s source of Bowser’s invincibility is specifically the Star Rod.