Tamatebako

The Tamatebako is a box that contains the power to make anyone grow older, and is rumored to be in the possession of the royal family and is somewhere in Ryugu Palace.

Vander Decken IX once planned to use the box to make a young Shirahoshi older in order to have her age match his so he can marry her. However, Decken abandoned the plan when he learned that there was no way for him to enter the palace. The box is not yet seen in the series, but was first mentioned by the Flying Pirates in Jinbe's flashback.

After the defeat of Hody and Decken, the soldiers discover that all the palace treasures and the Tamatebako, a national treasure, have been stolen.

It is later revealed that the Tamatebako was also rumored to "make one as strong as a thousand men". Both of these rumors are true, as the box contained the Energy Steroid, which give the users incredible strength, but after a period of time age them horribly.

Trivia

 * The Tamatebako is the mystical box in the legend of Urashima Tarō, where the titular character was gifted the box by Otohime upon leaving the Ryūgū-jō, but was told never to open the box. Urashima, upon returning to the surface world, realized that 300 years had passed, and in depression and curiosity, he opened the box to have a white smoke envelop him, which was Urashima's age that Otohime stored inside the box; Urashima instantly grew 300 years old.