Steel of the Heavenly Mine

A special holy metal used by god  to forge his sword in order to kill the eight-headed. The Boukengers tracked down the mine where they initially encounter the Dark Shadow, which ends up making them lose Natsuki. She eventually encounters a girl claiming to be her younger sister (in actuality the Tsukumogami Nendogami) and who tried to trick her into helping her get the metal from the mine. Though the Boukengers attempt to seal the mine off, they do eventually gain the ore which the Boukengers reclaim once Natsuki rejoins them and Nendogami is exposed and defeated.