The Ethereal Numerator can only be found in the Dream World only during Lunar Diplomacy.
The Ethereal Numerator directs one of the tasks the player must complete in the dream section of the quest. In "Communicating in numbers," the Numerator tells the player a sequence of numbers. The player looks for the pattern in the sequence, and then presses the two numbers that would come next in the pattern. This puzzle is fairly easy.
