Lost Girl

What you find in a cavern is a motionless girl who looks like she wants rescuing. She is not a townsfolk and there is no rescue. Touch the trigger and she becomes the Nymph, which hits harder than anything else you will meet before Hardmode.

The disguise does real work. Minions and homing weapons lock onto the passive form the same as any other enemy, so an auto-targeting build can spring the trap from off screen while you are looking somewhere else.

• Losing any health at all, from any source.
• Getting close with a clear line of sight, roughly a dozen tiles.
• Being moved in any direction, which includes falling and knockback, so breaking the floor under her is enough on its own.

The passive form ignores Confused. The transformed version does not, which is one of the cleaner ways to keep her off you once the fight has started.

She closes ground fast and does everything with contact damage. No projectiles, no ranged option at all, and she cannot jump high. That combination gives away the whole encounter: stand above her, or on the far side of a gap, and shoot. From a ledge she is harmless.

If you would rather do it up close, she is light enough to be stunlocked. The Harpoon is the standout because it fires fast enough at short range to pin her in place until she dies, with no risk to you. Flails, boomerangs, yoyos and the Terragrim all do the same job.

If one never turns up naturally, a lava farm with dart traps built once you have the Mechanic will eventually deliver her to you instead.

Her reputation as one of the rarest things in the game is partly an illusion. She spawns more often than people notice, but she does not move, does not make noise, and tends to sit in unlit corners, so you walk straight past her. Run a Lifeform Analyzer for an hour and the actual rate stops looking rare at all.

A Hunter Potion is the cheap version and it also gives away the trick, since it highlights her in the enemy color rather than the friendly one. Water Candles and Battle Potions raise the roll the same way they do for anything else.

Before you own an analyzer, the Magic Lantern light pet works as a detector by accident. It reacts to her as if she were buried treasure and makes her sparkle, which is a false positive you can use on purpose within a short radius.

On the Remix seed she moves to the Underground layer rather than the Cavern, so adjust where you spend your searching time.

The harmless-looking form has more defense than the monster it becomes, which is backwards from what anyone expects. Her banner shows both halves of the act, one on each side, and the achievement image uses the disguise rather than the real thing.

In Greek folklore a nymph is a minor nature deity. The Dryad is built on the same visual idea, just greener and wrapped in vines, which fits since dryads are a kind of nymph.