Crescent is the exact opposite of Sunrise, Amaterasu and Chibiterasu's trademark brush skill. It is very easy to use; simply drawing a curved line (to put it more simply, a backward "C") in the sky will reveal the crescent moon and change day into night. If it is already nighttime, Crescent will start the evening over again following the sunset.
Crescent can be used to access certain events, which only occur at night, quickly without having to wait for night to occur naturally, such as side-quests, such as the Hayazo side-quest.
After helping Kokari to catch Whopper, Yumigami appeared before Amaterasu and taught her the Crescent technique.
Amaterasu is required to use this technique twice in her adventure. The first is during the battle with Orochi. When Susano asked the gods to lend him the power to destroy the demon, Amaterasu used crescent to grant the strength to do so. The light from the moon gave his sword incredible power that allowed him to destroy the demon. Amaterasu used it in an almost identical situation when she traveled through the Spirit Gate. This time she used Crescent to empower Nagi's blade, Tsukuyomi, and gave him the power to slay Orochi.
Spoilers end here.
The power is named after the crescent shape the moon takes on at the beginning and end of each lunar phase.
If Amaterasu uses Crescent in the final battle with Yami, a giant spiritual apparition of Nagi will appear and slash Yami, splitting him in half and leaving his core open for attack.
When Crescent is used, the kanji 宵 ("yoi", evening) appears.