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Every 50th kill of this enemy drops Arapaima Banner.where it waits
Hardmode turns every pool scattered through the jungle, surface and underground alike, into a hazard. Nothing summons it, it simply starts spawning in the water once the world has shifted, and the caves below are full of small flooded pockets you have to swim through to get anywhere.
fighting it
It swims far faster than you do, it hits hard for something you meet this soon after the world changes, and it will launch itself clear of the surface to reach you on the bank. Standing next to the water is not safe ground.
Knockback works on it normally, and that is the whole opening. Anything with real shove keeps it in the pool and off your platform, and catching it in the air usually sends it straight back down. If you plan to work an area for a while, draining the pool or walling it off first is worth the few minutes.
trivia
Real arapaima are among the largest freshwater fish anywhere and have to surface to breathe, since the South American rivers they live in carry very little oxygen.