The Imperial scout using the key crystal.

"Besides being worth a fortune as an artifact, the crystal is also capable of boosting any sort of energy source, even mental energy!"
―Unidentified Imperial scout[src]

The key crystal was a Seoularian crystal which had control over all the other crystals. It responded directly to mental powers and could create shockwaves of increasing intensity. A short time before the outbreak of the Galactic Civil War, an Imperial scout discovered the planet Seoul 5 and removed the key crystal from the Crystal control room. He eventually lost the crystal in a card game to Han Solo, who brought the key crystal back to Seoul 5. Solo used the crystal to defeat Imperial Adjutant Ssssk!, but afterwards, he inadvertently caused the other Seoularian crystals to overload. In his hurry to escape the impending destruction, Solo left the key crystal and it was destroyed in the resulting explosion.