Originally posted by Draconifors:

"The forest is moving" is not the first one; it only starts occurring after you defeat Eikthyr.
The first one is "Eikthyr rallies the creatures of the forest", which is basically just a free food delivery.

After defeating the Elder you can get both "The ground is shaking" and "A foul smell from the swamp" (only one at a time, of course).

I believe that after you kill the swamp boss you will get "Skeleton surprise". After that, no idea.