There are several NPCs in Terraria that you can unlock by meeting certain requirements. These NPCs come to live alongside you, and if you trade with them, they provide you several useful items for you to purchase if you unlock them. A new NPC added to the game following the 1.4.1 update is the Princess, a regal individual who wishes to grace your game with her presence. But how do you unlock her?
You need to defeat the Queen Slime to unlock her. To summon the Queen Slime, you need to consume the Gelatin Crystal in the Hallow region. You can obtain the Gelatin Crystal by destroying pink ice blocks in the underground hallow region.
The Queen Slime will arrive shortly after you consume the Gelatin Crystal, and you can use it at any time of the day. You do not need to wait for any particular events to use properly to use the item. Queen Slime summons various minions to use against you, and when she reaches half health, the Queen sprouts wings, flying around the map.
The Princess NPC will arrive in your town after defeating the Queen Slime. She requires a home to be ready for her after killing the Queen Slime to summon her. Like other NPCs who make their home in your town, she sells various items you can purchase during your adventures.