The area deeper in Rocky Mountain Golem Cave (called "Deep area of Golem Cave" in WS1) is the underground extension of Rocky Mountain Golem Cave. It appears in both Witch Spring and Witch Spring 2. It is also where Blast, one of the Elemental Spirits, lives.
In WS1, Pieberry needs Lighting Bell set as her pet before she will enter the first time. After that, she shouldn't need him, but a bug may occur that forces you to equip him every time you go there.
Alfredo (WS2, Chapter 4): Upon entering after being invited by Pieberry to hunt Lavastein, Luna will hear his cry for help. A series of cutscenes and conversations between Luna and Alfredo will occur.
Hunting with Pieberry (WS2, Chapter 4): Whether you win or lose to her in racing, she'll be there in front of Lavastein.
Notes[]
In WS2, there is a gap to get to Lava that Luna will have to jump. Non-mount pets will have the mind-control spell lifted, though that can be circumvented by just re-summoning them after you reach the other side (or just use a mount pet, which will cross with her).
Due to the event with Pieberry, Black Joe cannot be summoned as a pet here even after the event has been completed.