The cup must be filled with bat blood at sundown then drunk while repeating the sacred bat-chant ("nana, nana, nana, nana") of the Bat Cult.
If successful, the person who drunk the blood now has the power of echolocation between the hours of sun-down and sun-up, which allows them to emit high piched bat-squeeks and 'see' the sound as it bounces back to their now bat-tuned ears. The potion must be drunk at preciesly sundown, or else the result it little more than nausea and a 30% chance of contracting rabies.
If the potion in inhumed properly, the person (ie, the player) must now speak in a low, hoarse whisper. Failing to do so will result in the person accidentally deafening/(and therefore blinding) themself for approximately 1d4 hours.