This week the lesson will be about the dragon blade!

The dragon blade is a blade that has the power to trap human souls.

We saw the dragon blade in the episode "Enter The Demon" (season 4). Yen-Lo was once the disciple of the zen master but used his knowledge for evil so the zen master passed him over for master so he swore vengeance. The only way to stop Yen-Lo was the dragon blade as it would trap his soul inside the blade.
The sisters helped the zen master's daughter to free her father who Yen-Lo captured in Limbo. Yen-Lo was trapped in the dragon blade at the end of the episode, all thanks to Phoebe.