"What the hell!? ...Is this the friggin' boss?"
During the second full moon (May 9th) after the arrival of the protagonist, Mitsuru Kirijo detects a powerful shadow outside of Tartarus and quickly dispatches the protagonist, Yukari Takeba and Junpei Iori to the monorail in Iwatodai Station.
After reaching the monorail's end they find a huge Shadow that has taken over the monorail, but after defeating it the monorail won't stop, leaving the protagonist to use the brakes manually, saving both Yukari and Junpei.
There is a time limit of 8 real-life minutes for the team to both reach it from the 3rd to last cabin and defeat it at the end of the monorail. Therefore, it is not advised for the player to take too much time at the Shadows in front of it or stay idle in the cabins, or they might end up having not enough time to defeat the boss. In harder difficulties, the time to cross the train and defeat the Priestess becomes shorter.
As all Shadows appearing on Full Moon nights, its appearance and behavior are based on Tarot cards from the Major Arcana in the reverse position. It has the appearance of a young woman wearing only the bottom half of a dress, her whole body is covered in either white or black, with the letters B and J on her breasts. Her hair looks like scrolls with text written on it, which are stuck to the wall not allowing anyone to pass. She wears a red, butterfly-like eyemask on her face, indicating her Priestess arcana. She herself sits inside the train on the floor with her legs spread out. She also seems to be abnormally tall at full height, even when compared to other Shadows.
As this is the Reversed version of the Priestess Arcana, it represents the dark side of the Arcana, very much like Shadow Yukiko. The Priestess means someone with knowledge and wisdom, yet the reversed version of it means someone unable to use that knowledge or someone not able to make decisions. Thus instead of confidently standing like the priestess does in the upright position, it instead lays on the ground sloppily.
The B and J on her breasts are taken directly from the Tarot card, they represent Boaz and Jachin, two copper pillars, which stood in the porch of Solomon's Temple. Notice how the Arcana Priestess summons two Muttering Tiaras during battle to aid her.
The Arcana Priestess takes over the train and attempts to cause a collision with another train. This is based around the conflict of intellect and instinct the card represents, which is shown with the Black and White coloration of the Shadow.
Also worthy to note, even after the Arcana Priestess is defeated the train still has to be stopped. Upon completing this, the protagonist is asked by the rest of the team how he knew which lever to use. To which he can either reply that he used his instinct or he took a lucky guess. Replying that it was instinct gives a much more positive response from the rest of the team as powerful instincts is an energy of the High Priestess card in the upright position.