Hidden Pit

This 10-foot-deep pit has a hinged lid constructed from material identical to the surrounding floor. When a creature moves onto the lid, it swings open like a trapdoor, causing the creature to fall into the pit. The lid remains open thereafter.

A creature that falls into the pit takes 3 (1d6) Bludgeoning damage from the fall.

Detect and Disarm

As a Study action, a creature can examine the section of floor that forms the pit's lid and make a DC 15 Intelligence (Investigation) check, detecting the pit on a successful check. Once the pit is detected, an Iron Spike or a similar object can be wedged between the pit's lid and the surrounding floor to prevent the lid from opening, thereby making it safe to cross. The cover can also be held shut using Arcane Lock or similar magic.

Escape

A creature needs a Climb Speed, climbing gear, or magic such as Spider Climb to scale the pit's smooth walls. (You can make the pit easier to escape by adding cracks in the walls big enough to serve as handholds and footholds.)

At Higher Levels

You can scale the trap for higher levels by increasing the pit's depth and damage, as shown in the following table.

5–10

30 feet

10 (3d6)

11–16

60 feet

21 (6d6)

17–20

120 feet

42 (12d6)