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Defeat the remaining Majin (Pale Rider, Dante, Mother Harlot, and Trumpeter) and place the Menorahs of Knowledge, Diadem, Beauty, and Divine Might on the dais to gain access to the fourth and fifth Calpas of the Amara Deep Zone.

At the entrance to the fourth Calpa, a Shinentai (an NPC (non-player character — anyone the game controls, not you) who provides information) warns you not to turn left. Following this advice avoids a corridor that is entirely Cursed (a status effect that damages you each step).

The cursed corridor contains 50,000 Makka (in-game currency) and a Deathstone. You can retrieve these now or wait until the cursed zones disappear after clearing the Calpa. The corridor also contains Graveyard (a location with specific enemies like Sakahagi and Spectre) and another Shinentai.

This second Shinentai will inform you about the "Eternal 12m," a corridor that leads to different places depending on the phase of Kagutsuchi (the in-game moon cycle). It warns against entering during a Silent Kagutsuchi, as you will end up in the Demon Lord's room.

In the north section of the first floor, a Dark Broker (an NPC who sells items) sells a Level 60 Mothman demon for 50,000 Makka. This demon has skills including all four Maha-element-Dain spells, Mahanma-on, Mahamudo-on, Megidoraon, and Chakra Step.

The ladder down to the next level leads to the "Eternal 12m."

The "Eternal 12m" is a unique area where you cannot use your map (a visual aid showing the surrounding area). It connects to four different locations: The Meifu Road, the Meifu Treasury, the Meifu Corridor, and the Meifu Plaza. (The Meifu is the Japanese land of the dead).

All four of these connected areas lead to the same place: the other side of the "Eternal 12m."

Beyond the "Eternal 12m" is a short corridor with a blocked-off entrance to the fifth Calpa. A Shinentai will explain that the switch to open this entrance is accessible from the Calpa above, specifically from behind a golden door that you cannot currently pass through.

After completing this Calpa, you will be able to find the key to that golden door.