The Pyramids

The Pyramids are pyramid-shaped buildings located in each of the four cardinal directions from The Bunker. They each contain a challenge, which produces a single purple cube of currently unknown use upon completion.

North Pyramid
The North Pyramid is in The City. Its challenge involved a trial to find a "traitor" character, noted by differences between said character and their canon iteration. Rae was eventually determined the traitor; the mastermind appeared to have planned to execute them, but decided not to at the last moment.

South Pyramid
The South Pyramid is on the Central Island. Its challenge was the "Wasteland Killing Game": one character must kill another, after which a trial will be held and the convicted character killed. In the game, George Smith killed Hiagi Yanekki, framing Morgan Fleaswallow. He succeeded, Morgan being killed by the mastermind.

East Pyramid
The East Pyramid is in the Life Pond in the Scarlet Forest. Its challenge is relatively simple compared to the others, requesting the sacrifice of two characters' lives and promising access to the Pyramid and restoration of the Life Pond in return. Rae and Erin were sacrificed, Rae committing suicide and Erin being murdered by Lilith.

West Pyramid
The West Pyramid is in the Crystal Caves. It's said to contain some sort of battle gauntlet, although it hasn't been explored by the Main Wave yet. Asgore Dreemurr claimed that it was incredibly deadly, allowing very few of his teammates to survive.