Level 3 Blaster
A giant boulder rolls through the chamber.Trap: The boulder rolls over everything in its path.Perception DC 10: The character notices the approaching boulder.Initiative +5 Speed 8TriggerAs soon as the encounter begins, the trap rolls initiative.AttackOpportunity Action Melee
Target: Attacks any character in a space it entersAttack: +7 vs. ReflexHit: 2d6 damage and knock prone.Special: If the boulder ends its turn in a space with a character, it makes a second attack. The character then moves to any open adjacent space (character’s choice).Countermeasures Athletics DC 20: A character can attempt an Athletics check as an immediate reaction to leap over the rolling boulder.
Published in Dungeon Master's Guide, page(s) 217.