The Dice of Death were used by the Killer Clowns.
Episode 2 - The Clowns Here Kill Part 2
The Dice of Death were used to choose someone to die (so if 1 to 10 rolled then someone would die). But if the Killer Clowns role a 12, someone gets freed from the Marry-Go-Round. When an 8 is rolled, Nikita screams, thinking she was gonna die, but Mortimer comes and saves her, along with the rest of the group.