Quick paintover I did. It'd look nicer if a) my tablet worked and b) it wasn't 1:30am. ^^
The only things I changed were the rotation of the hill and the color of the backdrop. It makes it seem more of a pit that way and helps add some contrast to the foreground/background. The added rotation/height probably isn't even necessary; I just wanted to see what it'd look like. I think the player gets used to seeing straight horizonal ground plane, so when you add in some diversity it demands extra attention and they might stop to think about why the terrain is changing like this.
I'd say you don't need to sell the idea that falling into a pit is a bad thing; just make the pit visible to the player and it should be evident that it's a bad thing to do. If they die it (should) only happen once, so no big deal. They'll just take it as a lesson learned.