Pyramids are large structures that generate in Atum. They are Atum's main dungeon and host to its only boss, the Pharaoh. As such, they can be very difficult for players who are unprepared or undergeared.
Entering the Pyramid
Pyramids can generate above or far, far below the sands. Blocks cannot be broken or placed on or in a pyramid until the Pharaoh is killed. All pyramids have a door on one side and two Quandry blocks next to the door. Nebu torches need to be placed on both Quandry blocks to unlock the entrance.
All pyramids have the same first floor, composed of several pillars, three limestone chests, and two Atum undead spawners. There are several unlit limestone torches that can be lit by interacting with them using a torch. Opposite the entrance is a ladder leading to the second floor.
The second floor is a maze, filled with four types of traps: Blazing (Fire), Poison (Poison), Tar (Slow), and Smoke (Blind). These traps cannot be broken, but when right-clicked with any pickaxe, the trap will be permanently disabled. allowing you to walk past safely. There are always two ladders, normally near the middle of the floor, that lead downwards to the final floor.
The last floor is also similar for all pyramids. There are 7 rooms, each off to the sides of the pyramid, containing random loot inside of crates. The two small rooms, closest to the Pharaoh's, have a spawner for a random Atum undead mob. Every room and the hallways can have random traps laid throughout, with a new arrow trap on this floor that shoots arrows as you pass. Directly behind where you come down from the second floor is the Pharaoh's room, with their Sarcophagus. Four Nebu torches are required to be placed onto the Quandry blocks in order to spawn the Pharaoh by right clicking the Sarchophagus (occasionally unlit Nebu torches will already be on a Quandry block, up to three).
After the Pharaoh
When the Pharaoh is killed, all traps will turn into normal blocks. All blocks in the pyramid will now be able to be broken. A lot of the Limestone Bricks will crack and fall to the floor, opening up holes both between floors and to the desert outside. Beware, as this can let in bandits or undead as you're trying to leave. You will also receive the Marked debuff for just over a minute, allowing Assassins to spawn.
All pyramids have three limestone chests on the first floor and several crates or chests on the third, with randomized (and often rather useless if you are at the point of comfortably killing a Pharaoh) loot. By far the biggest treasure is from the Pharaoh themselves, who will always drop an artifact, and there's an extremely small (.1%) chance of a crate or limestone chest containing an artifact as well.