The Night Elixir is a potion from Shadowgate 64: Trials of the Four Towers. This potion is used once early on in the game, but it can certainly come in handy later on. Much like the blue ring, when the main character, Del Cottonwood, drinks this elixir, his controls will become reversed. The reversed controls, however, only last until Del enters another room.
In the room adjacent to the room with a key-making machine, there is an empty bowl on top of a desk. When the Night Elixir is combined with Liquid Sunset in this bowl, the chemicals will change into a neon purple color and smoke will rise, revealing a key hole in the ceiling. When placing the key made from the key-making machine into this hole, the door with the moon image on it will become unlocked. This door leads to the room that contains the blue ring, the green ring, and the Ring of the Dead.
"You drink the liquid. It has no particular taste but it makes you feel quite odd." -narration upon drinking the potion
In the "reversed controls trial" in the Trials Tower, Del's controls will become reversed. However, drinking the Night Elixir (or putting on the blue ring) will counteract the reversed controls and allow the player to move around the course normally.