Rolling boulders are a dungeon staple, and their only drawback is that they are a one-shot deal: once evaded they lose their effectiveness. Magic can be used to upgrade the basic design. To see a good example of magic augmenting a trap, take a look at the ball of tentacles trap is a good example. This trap could be even better if the boulder could actively pursue its targets.
The crushing iceball trap fulfills that dream: it pursues, engulfs, suffocates, freezes, and crushes all intruders. It does that all with a cool efficiency that warms a trapsmith’s heart.
The crushing iceball trap is usually placed in long, 10-foot wide corridors. Particularly merciless trapsmiths employ portcullis traps behind and in front of the PCs to ensure that there is no escape and that death is inevitable.
Triggering the trap summons an aqueous orb in front of the PCs. The orb will roll forward and try to engulf the PCs. All PCs in its path take 2d6 nonlethal damage, and those who fail a DC 16 Reflex saving throw are engulfed by it and are carried along by it. These poor souls must hold their breath while inside the watery orb. At the beginning of each round, those inside the aqueous orb take 2d6 nonlethal damage. While the aqueous orb remains watery, a captured PC can attempt a DC 16 Reflex saving throw to escape the orb.
If the aqueous orb is successful in capturing one PC or more, the second part of the trap triggers. A freezing sphere strikes the aqueous orb. All within a 40-ft radius of the orb take 11d6 cold damage; a DC 19 Reflex save halves this damage. The freezing sphere also freezes the aqueous orb to a depth of 6 inches. Those seeking to escape the orb now require a DC 25 Strength check. Once the orb is frozen, it can no longer engulf creatures, but it can crush them! Creatures in its path can attempt a DC 16 Reflex saving throw to avoid taking 4d6 crushing damage. The aqueous orb moves at a speed of 30 feet and remains active for 11 rounds.
Crushing Iceball Trap CR 9
Type magical; Perception DC 31; Disable Device DC 31
Trigger sight (true seeing); Reset automatic
Effect spell effect (heightened, aqueous orb, DC 16 Reflex save negates)
Effect spell effect (freezing sphere, DC 19 Reflex save for half damage)
Effect crushing iceball (4d6 damage, DC 16 Reflex save negates; multiple targets)
Adding the aforementioned portcullis to the crushing iceball trap should raise the CR of the entire encounter by 1 or 2.