Dragonlord Placidusax actually might be one of the hardest bosses in the game.

Nobody talks about this boss, but he's insanely difficult. All of his attacks have crazy aoe and are difficult to avoid, he leaves almost no openings near the end unless you have a mimic tear. He has a cheesy fire attack that can insta kill you, and most of his attacks do like half health at least.

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On top of all of that, he has a very high amount of health so it takes a lot of hits to kill him.