Nymph

The Nymph is a rare enemy that spawns underground. When first encountered, it appears as the Lost Girl, who pretends to be a helpless NPC that needs to be saved. When a player gets close, it transforms and attacks as the Nymph, a dangerous enemy that deals high damage (for a pre-Hardmode enemy). After the first encounter it won't appear as a Lost Girl again. It is arguably the rarest creature in Terraria and is extremely hard to find.

Tips

 * It cannot attack from a distance, nor can it jump very high. Use a ranged weapon from an out-of-reach position.
 * A Harpoon or Flail can effective, as Nymphs can be stun-locked.

Trivia

 * The Lost Girl appears completely naked.
 * In Greek and Latin mythology, a nymph is a lesser female deity with the power to animate nature.
 * Could be inspired by numerous characters in popular culture that bait heroes by appearing harmless and in need of assistance.
 * She bears a striking resemblance to the Great Fairy in Legend of Zelda Twilight princess.