The Cabinet of Souls was an artefact of the Rhodian people, described by the Twelfth Doctor as the centre of their religion. It was said to contain a paradise where the souls of all Rhodians reside after death. The Cabinet could also be used as a weapon, as the power of all the souls within, compressed into specks of light, were capable of effecting genocide on another species.
When the Shadow Kin attacked Rhodia, the Rhodian prince Charlie took the Cabinet with him as he fled to Earth. The Shadow King Corakinus pursued Charlie so as to prevent him from using the Cabinet to wipe out the Shadow Kin. Charlie later claimed to Corakinus that the Cabinet was empty, that its legend was merely a bedtime story for children to make death less scary and that he was only keeping the Cabinet in memory of his dead people. However, Charlie lied as there were in fact multitudes of souls inside the Cabinet. (TV: For Tonight We Might Die)