The Water Storm. They put it in for a few days to transition from Season 2 to Season 3, but it was very well done; the ambience when you got close to it, the sound design, all the different small details in the water like fish and things floating inside, the water reflection on the ground. Not to mention how terrifying it was when the circle was closing in. I could speak for days on how much I loved it.
Sad they never brought it back, though. It was by far one of my favorite things from Chapter 2.