Quest
Where to find the Sage
Start in the Main Hall, not the tomb
The Sage is Eleanor of Aquitaine. She is imprisoned in a mirror on the southeast side of the Main Hall. You reach her by breaking the four sage sigils. The Hall of Mirrors page lists those four rooms. Holding down at a sealed mirror is the entry.
Once the hall is open she is taken by The Temptress. Beating the Succubus in Belmont Manor does not hand you the card. Go to Holy Innocents' Cemetery and open the Tomb of the Noble Queen. That is where the Sage arcana unlocks.
Why the card matters
The Sage is one of the Four Virtues required for the good ending. The others are Courage from Joan's rematch, the Whisperer line through Mammon and the ferryman, and the Judge. The Self, from the Doppelganger, is separate and also required.
What I ran into
The first answer I drafted sent people to the tomb. That is the last step. The first screen now says Main Hall, because that is where I actually started looking after I reread GameSpot.
Questions
Where is the Sage at the start?
In a mirror on the southeast side of the Main Hall. Break the four seals to reach that mirror.
Where is the Sage card?
Tomb of the Noble Queen, after The Temptress is dead.