The Butterfly Pool is a small body of water in a cavern located on the second level of the Magisterium's dungeons. The lake is fed from a spring aboveground and pools down into a lake in the cave. The water is mineral-rich; as a result, it glows in different hues of blue and is ideal for mages to craft their weapons with.
During their Iron Year, Master Rufus sent Call, Tamara, and Aaron down to the pool to collect some water and bring them back intact with magic. Tamara guessed that the pool got its name from blue monarch butterflies.