A potion made from the essences in Roper Klacks' laboratory. When drunk, it makes April invisible for a short period of time. April says that it would be perfect for sneaking into clubs back in Newport.
One of the traps in Roper Klacks' labyrinth is a mirror which, when April looks into it, shows a dark, 'mirror universe' version of herself that reaches out from the mirror and tries to grab her. April needs to retrieve a page lying in front of the mirror, so she drinks the potion to ensure she won't cast a reflection and manages to get it.
There is still a little of the potion left, which comes in handy when April is trying to escape the Vanguard at the Border House. After she jumps into the canal from her window, she needs to escape the Vanguard sentry by the front door, so she drinks the potion to sneak past. However, the sentry can still sense April's presence, so the potion only masks a person's physical form, not the sound of their movements.
Mixed in Roper Klacks' laboratory.