Hyrule has a few dark secrets in The Legend of Zelda: Tears of the Kingdom, and many of them are pretty sinister. Even beyond the entire Depths section that lies below the ground, there are a lot of different things that you can stumble across that can give you chills.
The quest, Who Goes There? is such a situation that has Link interacting with one of the game's darker content. A quest that has you investigating an eerie voice that leads you to something completely different from what you may think.
Updated by Shane Black on June 12th, 2023: Added some more inter-text links to relevant guides.
How To Start Who Goes There?
You can find this quest in Lookout Landing after you have completed one of the game's regional temples. When you have done so, head down into the bunker underneath the town, and you can find Jerrin, a woman who is frantically trying to clean up the area in front of a large hole in the wall.
Speaking with her will reveal that during her cleaning, she accidentally put a hole in the wall, and ever since, she has been hearing a creepy voice coming from the hole. She is convinced it is a demon and that her days are numbered, so you can offer to help her find the source of the voice and hopefully quell her fears.
How To Complete Who Goes There?
The quest is super simple and starts by going through the hole in the wall. On the other side, you will encounter tunnels that are blocked by rocks.
You can use either a weapon fused to a rock of some kind to break it apart or use a bomb flower on its own or with an arrow. Either way, start clearing out the rocks and keep to the right.
There will be a fork in the tunnel, and you need to go to the right one, which will reveal to you a sinister-looking statue. You will notice speech bubbles coming off of it, saying some not-so-fun things, so it stands to reason that this is the source of the voice that Jerrin has.
Walk up to the statue and have a conversation with it. While not a demon, you will learn that the person who is trapped in this statue is not a good person, necessarily. They were someone who went against the will of the goddess and had gone through a series of punishments, culminating in being trapped in this current form.
The statue - and its formerly-human inhabitant - are part of the A Deal with the Statue quest, and long story short: the statue offers you a way of exchanging money to give yourself more hearts or stamina.
Once your conversation with them is done, head back to the bunker and speak with Jerrin. She will thank you for helping ease her fear and comment on how the statue's existence is to balance the light and the dark of Hyrule. But that's a more philosophical question for a different time, so once you have the quest completed graphic, you can leave.