Floaty Gross

It spawns by itself in the Underground Crimson once the world has gone hardmode, but under stricter conditions than most enemies. It wants no town NPC nearby and no safe background wall behind you, and it wants some sign of life from you in the last few seconds: a step, a jump, a key press, an item used. Standing still in a walled off room genuinely keeps it away. Holding a Shadow Candle, or standing near a placed one, cancels every one of those checks, which makes the candle the right tool if you are hunting one.

It floats through solid ground the way a Wraith does, which sounds unanswerable until you notice how badly it takes knockback. A heavy weapon shoves it deep into stone, and in the tight passages of the Underground Crimson that means a long swim back out. Light fast weapons let it keep coming.

Contact can leave you Weak, and a defense cut lasting minutes follows you into the next fight. That is the real cost of touching one.