Turns out the dragon started "dive bombing" us. Lydia was taking the brunt of it, working her way deeper into the ravine, where the passage narrows and the walls are even higher. The dragon had at least 80% of its health still available. I was a couple dozen meters away and Lydia was just rounding a corner in the ravine. The dragon swooped down on another nose dive... almost straight down....
And then the music stopped. I walked over and, sure enough, the dragon is laying there dead as a coffin pilot. I'm level 14 so its not like Lydia and I are wearing dragon swatting gear. I swear the dragon hit the ground and died.
Is this possible?!
