Azoth

One of the most important symbols to alchemy, also known as the "philosopher's stone".
In the game, it is required in order to create artificial life, making it so to speak the essence of life - perhaps it could even be rephrased as being a "soul".

Azoth itself is the "lifeforce" each living thing has inside its body. Since the Azoth inside Fiona is a special kind, directly descended from that of the famed mediaeval alchemist Aureolus Belli, she is targeted.