Havrik Stoneskimmer
Adventurer
It's very vague. The exact text is "Each turn of slumber roll a d4, and if a 4 results, a stone Juggernaut (rather like a steam roller) comes out of the 20'x20' room to the north and rolls 1-6 spaces (10'-60') south then west as determined by a roll of d6".Huh?! I always took it that the Juggernaut rolls 10-60' EVERY turn if the DM rolls a 4.
If it wasn't for the "comes out of the 20'x20' room to the north" part, I might agree with your interpretation that the juggernaut keeps moving each turn. Although that itself is odd: it stops for 10 minutes and then starts rolling again?
I think the most literal interpretation of the text is that it's meant to be a one-shot trap that is completely lethal if you draw the unlucky numbers. Kind of a lame trap, IMO, although any other interpretation is even more unfair and lethal.
The alternate idea that the PCs could individually wake up while the trap is activating is really cool, but would require a lot of DM judgments that the module doesn't provide: Did you wake up before or after your friends got squished? Can you stop the juggernaut from moving? Can you drag your sleeping friends away in time? Even though it requires a lot of DM improvisation, I might run it this way just to turn it into a trap that could be fun instead of another case of "rocks fall, everyone dies" with no warning.