Blood Crawlers. I have gotten 4 banners from them just from making a BoC arena in the chasm.
Because not everyone plays the same and when you're on a hardcore run and it ends with the fiery of hell being reigned from a singular enemie's fingertips, it sucks.Not really understanding why considering that he's only found in one area of the game you've not much reason to go to for any extended period of time (once you get a few accessories or a weapon or two you're done there for the most part) and if you're there for ectoplasm you simply don't go to the appropriate background wall areas and you'll very rarely see him.
And even then, he's nowhere near the most annoying enemy in the dungeon and there are non-dungeon enemies you find everywhere that are harder/more damaging/etc.
Down in the underworld where you have almost 0 reason to go to Post-Hardmode, unless you want a Fire Feather, or the trident for some weird reason.WHERE'S RED DEVIL!?