The soul-eating jar, known in the Japanese version as the xing zhen hu (ja:
The card "Xing Zhen Hu" is based on this jar.
The Dragon Cards are usually kept in a seal. When the seal is broken the only way to reseal it is to offer the soul of the loser of a game of Dragon Cards to the dragons. When this happens the dragons get sucked back into the jar and pull the soul of the loser with them.
The dragons consume the souls of people dragged into the jar. This process takes three months and only one soul can be consumed at a time. If a second soul is pulled into the jar, the first one is released.