It is assumed that you are reading this after having explored every part of Yvelheim that you could get to. If not, get your lazy ass out of this page and start clicking.
Having spoken with the Great Green Yvelheim Wyrm, you have by now learned that he can tell you of the Key of Earth, or could if he weren't so hungry. And he is a vegetarian. Feeding him seems like the thing to do, but you have no food. Still, you know where you can find some. Wander over to the pasture where the blackberries grow, and pick some. The problem is that you have a tendency to slowly eat them while you are walking. If you take too long in getting back to the cave from the pasture, you'll finish them off and won't have any left for the Dragon. So you have to hurry. Go directly from the pasture to the road, follow the road, and head straight for the cave. You'll have some blackberries left for him. Your next task will become clear, though it will be trickier.