Lamia

Lamias live in one place, the underground desert, and which one you meet depends on what has spread over that desert. Untouched sand and Hallowed sand both give the pale haired version. Corrupt or Crimson sand gives a much darker one. They fight identically, but they do not share a drop table completely: the Sun Mask comes only off the pale one and the Moon Mask only off the dark one, so collecting both means finding underground desert on both sides of your world evil. In a Hardmode world that usually means checking each arm of the V before you commit to a farming spot. The vanity set drops at the same odds from either, and the dark version has one extra drop of its own beyond the mask.

In Greek myth Lamia was a queen of Libya whom Hera cursed into a monster that devoured children. Later fantasy writing reshaped her into the snake bodied woman that turns up everywhere now, and that is the version the game draws on. The banner, incidentally, shows the dark variant rather than the pale one.