Requiem was made by Nine to replace the Master Unit, and it was able to trigger a massive scaled Event Interference. Nine described it as "a perfect system that will prevent someone else from intervening so that anyone's dream can come true".
Nine wanted to rewind time back to the point where she could create a new world that is not based on Amaterasu's dreams. She also planned to kill Hades: Izanami with Requiem, and even though she was not able to complete it, she activated it during their fight. With Requiem's power, Nine was able to freeze time in the room where they were fighting, delaying the Day of Ruin and stopping Izanami for a week, but she used her own life as a catalyst to activate it.
Requiem went out of control, and it should have exploded as soon as time resumed, destroying the whole world. To prevent this, Kokonoe destroyed it by using Ex Machina: Minerva, whose power source is a miniaturized reactive furnace.