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Taking the shape of a black box, it can be used to expose a gaseous substance to a person, petrifying their entire body, resulting in them being preserved alive for ten-thousand years.

Etymology

The technique is named after "Ryūgū-jō", an undersea palace of Ryūjin. It also appears in the tale of Urashima Tarō, the same tale Urashiki is based on. Also, the box shape and its time freeze property could be an allusion to the box Urashima received in Ryūgū-jō, which later turned him into an old man after the centuries he spent in Ryūgū-jō without aging.