In Super Mario Odyssey, 500 Power Moons are required to unlock the Darker Side. The threshold applies after Mario has completed the main story and defeated Bowser, when the Odyssey can use the additional Moons to reach the final challenge area.
The Darker Side is a post-game kingdom located beyond the Moon Kingdom. Its main attraction is “Long Journey’s End,” a demanding obstacle course that combines mechanics and hazards from several earlier kingdoms. Unlike ordinary kingdoms, it has no conventional regional-coin economy and is designed as a final examination of the player’s movement skills.
A common mix-up is confusing the Darker Side with the Dark Side. The Dark Side becomes available at 250 Power Moons, while the Darker Side requires 500. The requirement counts Moons deposited into the Odyssey, including purchasable duplicate Moons from Crazy Cap shops, not merely unique discoveries. The game ultimately allows Mario to collect up to 999 Power Moons.