Have you installed the
patch? In an earlier version a couple people ran into problems with monsters not attacking, but that should be fixed.
The cut scene with the rock only happens after the alchemist asks you to get some ingredients for him.
In a previous version there was a bug with finding the ingredients and moving them in your inventory before talking to the alchemist. That would trigger a quest log update prematurely, and prevent the cut scene at the rock. The workaround for that problem may also work in this case;
[*] Complete all other quests and save the game.
[*] If you have a save before finding the alchemist's ingredients and moving them, copy the 2 story.* files from that save into your latest save, otherwise you can use
these files from a save at the beginning of the imp village.
[*] Talk to Altoflix in the north of the village (if required), then the chieftain's son to get the quest to find the alchemist, talk to the alchemist once (only) and head to the cave.
[*] Continue with the game.