Simple Pit Trap
The trap is poorly concealed and requires a successful DC 12 Wisdom (Perception) check to spot, but cannot be disarmed, only avoided by passing a DC 13 Dexterity (Acrobatics) check. A character unaware of the trap must make a DC 15 Dexterity saving throw to avoid it. On a failure, a character falls 40 feet through the floor into a freshly-dug pit.