Undertale.
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—A retired destroyer moves to another multiverse to live a peaceful life.
—Things go wrong for him once he decides: hey, i live diving to observe and collect things in the sea, plus i have the hobby to clear it too, why not go sleep for a night at a small island not too far away? Why not get stuck in a cave too!
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—After that something strange happens and now he has to escape ancient leviathans and merpeople that want to take him in.
—And avoid mers obsessed and slash or interested in him too.
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—There is even a weirdo named Bill who seems to be stalking him.
—He can't unsee the disney princess bullshit now.
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