NieR: Automata's 3C3C1D119440927 DLC unlocks three challenging Colosseums. After receiving the Mysterious Letter, you'll need to locate these arenas to earn unique rewards. Here's a guide to their locations:
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Trial of the Sand Colosseum Location
This Colosseum is situated in the desert. From the Desert: Center access point, look to the right (facing the direction you entered from) for an orange diamond marker. A machine guards the entrance, but it poses no significant threat. Completing this Colosseum offers several ranks, culminating in the Destroyer Outfit (A2) reward for achieving Rank S.
Gambler's Colosseum Location
Located in the Flooded City, access this Colosseum from the Flooded City: Coast access point. Follow the path towards the coast (the same route used to protect the resource ship). At the end, locate a waterfall to the right. Circle around the building to the left of the waterfall to find a Resistance member guarding the entrance. A 1,000G bribe unlocks access to the Gambler's Colosseum and its tiered challenges. A Rank S completion yields the Revealing Outfit (2B).
Underground Colosseum Location
This Colosseum is only accessible while playing as 9S. Starting from the Forest Zone: Center access point, follow the left edge of the forest until you find machines training near a large waterfall. Passing through the waterfall will grant entry, initiating the Underground Colosseum quest. The Young Man's Outfit (9S) awaits those who conquer this arena to Rank S.
