Quick, take a left, that giant rolling boulder can’t turn corners!
– Famous Last Words
A giant boulder that falls from its mount and begins chasing someone down a long, straight tunnel is so common that it’s almost a fantasy trope. And it’s a good one – there’s no clever escape, just you versus how much time the trap maker gave for the boulder to run down the hallway.
But for particular cruelness, you give that boulder a touch of sentience through magic, perhaps by means of a bound earth elemental. Just enough that when you dive out of the way, the boulder changes directions in order to crush you anyways.
Homing Boulder Trap CR 9
Type mechanical and magic; Perception DC 16; Disable Device DC 31
Trigger proximity (location); Duration 6 rounds; Reset none
Effect a large boulder begins rolling down a predetermined pathway at 20 feet per round. Characters struck by this boulder take 4d6 bludgeoning damage and are knocked prone. Every round, the boulder either accelerates by 20 feet, or change direction up to 90 degrees. Alternately, it can stop on a dime and resume rolling in any direction at 20 feet per round. The accelerations not only makes it possible to crush faster and faster characters, but increases the damage done if struck by the boulder by 2d6 points. Characters can make a DC 20 Reflex save when they would be struck by the boulder for half damage.
The boulder has 500 hp, and hardness 6.
For maximum effectiveness, the boulder should have at least 400 feet of straight corridor, or for maximum surprise in a large open space 200 feet to a side.