Dream Coffins ("Dream Boxes" in the English dub) appeared in the Sailor Moon SuperS movie, and took the form of a six-sided container made of a purple-hued transparent material. Queen Badiane trapped the children of the world inside of them in order to absorb their energy, using it to power her Black Dream Hole, and wanted every human to likewise be trapped in these coffins once her Black Dream Hole consumed the Earth.
Upon arrival at Marzipannu Castle, the kidnapped children ran off the gondola and were immediately trapped in these coffins. The coffins then moved through tubes into the heart of the Castle to feed the Black Dream Hole. When the Sailor Senshi arrived, they discovered that some of the coffins made up the floors of the main hall of the Castle.
Once Badiane was destroyed, the coffins separated from the Castle and disappeared, leaving behind the children sleeping inside.