Corrupt Slime

It belongs to Hardmode Corruption, both the purple strip on the surface and the chasms under it. On its own it is slow and easy to read, so nobody dies to one directly. What catches people is the second half of the kill, and the fact that these are rarely alone in the room.

Death splits it into a small cluster of slimelings that scatter at once, and each of those rolls the loot table on its own account. Anything you actually want off this enemy therefore comes at far better odds than the parent alone suggests. That covers gel in bulk, Souls of Night while you are down there anyway, and above all the Blindfold, which is a miserable hunt from any other direction.

Parent and fragments alike can blind you on contact, shrinking your light radius to almost nothing. Out in the open that is a nuisance. In a corruption chasm it is how you walk off a ledge you could no longer see, or lose track of the devourers coming at you sideways. Keep torches along whatever lane you fight in, or run a Shine Potion and stop thinking about it.

A Royal Gel makes the whole family ignore you, which is the cheapest safety you can bring to a corruption farm. The complication comes from the split. Fresh slimelings pass through solid tiles for a moment after they appear, so a lava pit that assumes its walls will hold them leaks, and any that survive tend to gather in one corner. A crowd sitting there counts against the spawn limit and slowly strangles the farm. Give the setup a second way to finish things off, away from the main pit, rather than trusting one trap to catch everything.