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. Each Colosseum features six ranks of progressively difficult enemies.
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Trial of the Sand Colosseum
Located in the desert, access this Colosseum from the Desert: Center access point. Look to the right (facing the desert entrance) for an orange diamond marking its location. A machine guards the entrance, but poses no significant threat. Completing Rank S rewards the Destroyer Outfit (A2).
Gambler's Colosseum
Starting from the Flooded City: Coast access point, follow the path towards the coast, similar to the route taken to protect the resource ship. At the end, locate a waterfall. Circle around the building to the left of the waterfall to find a Resistance member guarding the entrance. A 1,000G bribe grants access to the Gambler's Colosseum quest. Rank S completion rewards the Revealing Outfit (2B).
Underground Colosseum
9S only: This Colosseum is accessible only when playing as 9S. From the Forest Zone: Center access point, follow the forest's left edge until you find machines training near a large waterfall. Pass through the waterfall to enter. Completing Rank S rewards the Young Man's Outfit (9S).