Millennium Cave

The Millennium Cave is an ancient site known only to the Tailed-People Clan Jashinka Empire that is used for the punishment of traitors and criminals.

History
Eight years prior to the series, General Zenobia was sent to this site to be punished after her attempt to assassinate Emperor Aton due to her desire to be empress of the Jashinka. Though she was meant to remain within the confines, the disturbance around the cave after it was found by a human scientist was enough to awaken her and use her seven-tailed-power magical abilities to break free from within. Although she still had the black mark for what she did, Zenobia was welcomed back to the Jashinka due to the failures of the other members of Aton's court.

Under Zenobia's influence, the fault of the Jashinka's failure quickly migrated from her to the under-fire Prince Megiddo, who had held several failures on his record assisting in his father's cause. After another failure, Zenobia asked Aton to give judgement to Megiddo, who ultimately found him unfit for any further usage to the empire due to his constant failures. Zenobia has Megiddo punished after losing all his remaining tails by being sealed within the Millennium Cave, where he was forced to survive with no food and only the water dripping within as sustenance. While sealed within the cave, Megiddo swears revenge over and over, not only on his father and Zenobia but also on all who lead to his plight, including the Dynaman. Eventually using his own power and abilities, Megiddo mysteriously escapes from the back of the cave, where the other Jashinka saw him nowhere to be found when they checked his location.-

After the Jashinka eventually changed hands with the death of all major generals and Megiddo himself ascended to the emperor-ship, his first action was to have the five Dynaman sealed within the Millennium Cave, seeing them with no means of escape and without the abilities of the Jashinka. With no other means of finding escape,  the Dynaman decide to coordinate the auto-control of their Dyna Brace with carrier DyJupiter, seeing it as their only means of survival. With Dr. Yumeno piloting, DyJupiter ultimately reaches the site and uses its missiles to blow open the doors to the sealed cave, destroying its sealing power forever while helping the team escape.