The Infinite Cocoon was a device capable of modifying and healing biological organisms.
It was a marble-grey box about the size and shape of a coffin. One end had a console with 169 labelled white controls. The device could break down living matter and reconstitute it. Properly used, it could heal almost any injury or change a lifeform into an entirely different species. Only a few were ever created, and the Time Lords considered it so dangerous that one of them was placed on the Consolidator.
After the ship arrived at Praxilion, the Cocoon was obtained by the natives. The Red Queen believed it was the worst machine in the universe. However, she ordered it to be used to transform natives into humanoids so they could access more areas of the Consolidator. Many died during the experiments before success was achieved. One of the failures, Ver, was changed into a monster and had to be killed. Out of the 169 controls, they were only able to understand how to use 75 of them. The Third Doctor later used it to heal the Master from damage he had received from the Sild. It was also used to restore the Red Queen's health and youth. (PROSE: Harvest of Time)