Who is The Sacrifice for Salem?

Introduction
The idea of the "Sacrifice for Salem" is the central focus of the novel.

Set-up
The Sacrifice for Salem is first mentioned by The Wand of Water, in which it tells Mary that there must be "one more death" for them to succeed in their plan to stop Cassandra from coming back from the dead. The Wand, however, cannot tell Mary who this sacrifice is, as this could change the outcome.

Answer
The Sacrifice is, at first, implied to be Mary herself. She is taken captive by Joshua Hamilton, who tries to cut her throat at Salem Common. However, she is saved by the remaining members of The Salem Four, who stop Hamilton and defeat him.

Just as it looks like the sacrifice has been averted, Marcus and Beatrice arrive. Marcus holds a gun to Mary, as Beatrice begins a spell to bring the Witches of Old Salem back. Just as Marcus is about to shoot, an exhausted Florence throws herself in front of Mary, taking the bullet herself. She then falls to the ground, her blood soaking the ground and being used in the spell. Florence soon dies, making her the "Sacrifice for Salem".

Trivia

 * The original "sacrifice" was supposed to be Callum, but this was later changed when he was given a different role in the story.