Nothing in these videos is story related, but you do see the big guy and the final quest reward.
DO NOT WATCH/READ IF YOU DON'T WANT EPIC SPOILERS
(parental advisory: mature language is used in these videos)
First look at him: [url]http://www.youtube.com/watch?v=QRgch1RFyu4[/url]
After beating him: [url]http://www.youtube.com/watch?v=mKWh_eqPXRM[/url]
This is a transcript of a 'survey' from the Zhaitan boss fight. This includes something called the Mouth of Zhaitan. This seems to point toward one very important thing: The dragon in this video is either NOT Zhaitan (he's freaking small, c'mon) or he is a PART of Zhaitan. Zhaitan is supposedly made up of many smaller dragons, so this is not surprising.
EDIT: Found this as well: [url]http://cghub.com/images/view/289683/[/url]
If THAT is the mouth of Zhaitan... and the dragon in the video IS Zhaitan... I'll be disappointed.