The Tamatebako is a box that is said to contain the power to make anyone grow older. It was in the possession of the royal family of Fishman Island and was stored in the treasure room of 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 that he could marry her. However, Decken abandoned the plan when he learned that there was no way for him to enter the palace.
It 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, had 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 gives the user incredible strength, but if heavily abused, it ages them horribly. The Minister of the Right later told Neptune that he filled the box with bombs that will explode if the box is opened. He did this as a security measure since the contents of the box were stolen ten years ago.
The contents were stolen by Hody Jones and were mass produced by Zeo. They were primarily used to bolster the strength of the New Fishman Pirates, and upon seeing the officers' heavily aged appearances, Neptune came to realize that Hody was the thief of the contents. The box was later stolen by Caribou during the coup and was recovered by the Straw Hats. Luffy gave it to the officers of the Big Mom Pirates, Pekoms and Tamago, who later left with the box and the rest of the palace treasure. Sometime later, the box went into Big Mom's possession and she planned to open it during Sanji and Purin's wedding.
The Tamatebako is the mystical box in the legend of Urashima Tarō (a well known Japanese legend), 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.