Tenchi-ken is the name of the sword Yoshou used to defeat Ryouko in the past.
At the beginning of the first OAV series, it contains all three of
Ryouko's Power Gems embedded in its hilt,
effectively sealing Ryouko powerless in the cave.
When Tenchi first finds it, Tenchi-ken's blade is rusted and shatters. Later in the first episode, a blue energy blade similar to Ryouko's own Energy Sword is emitted from its hilt.
When introduced to Aeka and Sasami in episode 2, we are told that
Tenchi-ken is Yoshou's key to controlling the Juraian Trees. Each tree
has its own key, while Tenchi-ken was special as it had the power to control
any tree. Sasami and Aeka also have their own keys.
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