The Box of Ultimate Bliss (極楽の匣, Gokuraku no Hakō) is a giant box with a demonic mask-like face on each of the four sides that is found in Kusagakure.
The box was thought to be able to grant the person that opens it any wish, though, according to Gamabunta, this is simply a myth derived from people who were too desperate to put genuine efforts into their dreams, and in reality it simply corrupts whatever enters within its confines. The box has such powers that it once allowed Kusagakure to take over the world during the time of the Sage of Six Paths. Once news of it had resurfaced, all villages stop any conflict to deal with the box. It requires an immense amount of chakra to open.
Once opened, a monster named Satori, called the puppet of the box, comes out. The puppet's job is to capture enemies and throw them into the box. Enemies thrown into the box become comrades. If no one is controlling the puppet, it goes berserk and tries to suck everything into the box. The box closes when the puppet is forced back into the box or the medium is killed.
The box is virtually indestructible. Naruto tried destroying it in Sage Mode with a Big Ball Rasengan and later a sage chakra-enhanced punch and didn't even scratch the box, and Muku, who had been locked in the box for ten years, claimed that he couldn't destroy the box from the inside no matter what he attempted.
The guardian of the box, Mui, tried opening it once ten years ago with the chakra of his son, Muku, in the hopes of reviving the destroyed Kusagakure. Muku didn't have enough chakra and was sucked into the box instead, leaving his father grief-stricken.
Mui and the four council members of the Hōzuki Castle framed Naruto in order to gain access to Kurama's chakra to open the box, in the hopes of reviving Kusagakure with the wish that supposedly came with the feat; however, Mui had his secret agenda instead: to wish to see his son once more.
Eventually, Mui succeeded in reopening the box, and Muku stepped out, but the boy was possessed by Satori, and wreaked havoc by throwing prisoners and the four council members into the box. Eventually, when Satori was defeated, Muku sacrificed himself in order to seal the demon back into the box. Killer B then placed the box into the ocean current around the island of the Hōzuki Castle, so it would never be used again.