Bestiary+

[Manly Man]

Well, the Pumpkin Head spell could potentially be a thing. Something like doing 1d8 for every 5 hp below max you are, up to a total of, say, 10d8? You're going to have to at least have 51 hp to get that, which is a pretty hefty chunk of health gone, and would also justify it being a somewhat lower-level spell than most that could get that high of damage, since those just scale on their own without putting you in danger.