Pandora's Box was a thermonuclear doomsday device.
Butler and Swarf stole it from a top secret military installation and brought it to the seventh planet in the system. They were told it would be used to create an opening to another dimension which had large amounts of the valuable drug Dream B.
This was only partly true. The Fellowship wanted the opening, but only as a means to allow the Yssgaroth into our universe. Their plan was stopped by Spike, who blew up the machinery that was connecting to the other dimension.