This is the nine-tailed divine spirit form of Cas-ko. In this form she is a pseudo-god that is a deification of the Sun. You get a peek of her like this in her 3rd Secret Garden.
The apathetic Sun Goddess (Amaterasu Oomikami).
When Cas-ko goes into nine-tails mode this is what she will......not be. This form is Cas-ko’s origin, and is now just something that’s been part of her since long ago and continues to be a part of her.
When she becomes this high level of a divine spirit, the time axis is no longer a limiting factor to her.
Normally someone entering the mausoleum would be burnt to a crisp, but as expected of our Zabio/Zabiko [nicknames for the male/female protagonist] he or she flaunts their recklessness in the face of danger (stupid menu choices), and somehow pull through.
Originally we wanted to feature her without clothing (in her lovely nine-tailed divine spirit form wearing only an apron (naked-apron)), however unable to break through the ethical wall that is our consciences, she appears properly clad in her ceremonial attire.
By the way, when someone who has no dream enters the mausoleum the only thing that can be seen is a giant sun.