Ok, it's a getting-on-for three year old game and I daresay it's not actually a spoiler to many people on this forum, but I remember my surprise and wouldn't want to ruin that for anyone. So, if you haven't already met a character named Paarthurnax, I suggest you stop reading.
What was your reaction when you first discovered that Paarthurnax, leader of the peaceful Greybeards and who you are referred to when you need help to defeat Alduin, is a dragon?