Boss
Who The Prophetess is
One guardian, two names
The Prophetess is the card you absorb from Medusa. Konami lists her among the bosses with Melique Berger as the voice. The room is the Ossuary of the Innocents. She is the third guardian, and you can fight her before or after Joan on the tower.
The key she drops is The Prophetess's Key. Together with the keys from The Fallen and The Warrior, it opens the Catacombs of Paris. The spell on the card is Petrifying Gaze. Combat timing lives on the Medusa page so this one can answer the name.
What this page will not do
It will not paste the gaze and tail script a second time. If you are dying, use the Medusa page. If you are asking which guardian this is, or whether a second boss is hiding under the title, the answer is no: one arena, one key, one card.
What I ran into
I had both URLs in the plan and almost made them twins. The useful split is identity versus timing. I kept the timing on Medusa and used this page for the key and the voice credit.
Questions
Is The Prophetess a separate boss from Medusa?
No. Medusa is the fight. The Prophetess is the card and the guardian title.
What does the key do?
It is one of three guardian keys. All three open the Catacombs of Paris.