NieR: Automata's 3C3C1D119440927 DLC unlocks three challenging Colosseums. After receiving the Mysterious Letter, you'll need to locate these arenas to access unique rewards. Here's a breakdown of each location:
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Trial of the Sand Colosseum Location
This desert arena is accessible from the Desert: Center access point. Zoom out your view and look to the right (from your entry point) for an orange diamond marking its location. A machine guards the entrance, but it poses no significant threat. Completing this colosseum to Rank S rewards the Destroyer Outfit (A2).
Gambler's Colosseum Location
Located in the Flooded City, access this colosseum from the Flooded City: Coast access point. Follow the familiar path towards the coast (the route to the resource ship). At the end, locate a waterfall to your right. Circle around the building to the left of the waterfall, where you'll find a Resistance member guarding the entrance. A 1000G bribe unlocks the Gambler's Colosseum quest. Achieving Rank S grants the Revealing Outfit (2B).
Underground Colosseum Location
9S Only: This colosseum is exclusively accessible to 9S.
Start at the Forest Zone: Center access point and follow the forest's left edge until you encounter machines training near a large waterfall. Passing through the waterfall leads to the entrance, but only if you're playing as 9S. The Underground Colosseum quest begins here. Completing to Rank S rewards the Young Man's Outfit (9S).