I checked out the old official Majesty website, which is still online:

http://www.cyberlore.com/majesty/index.htm

https://web.archive.org/web/20010607201930/http://www.cyberlore.com/majesty/index.htm

Apparently the demo has a "unique quest" that's not in the final version. Is this the The Wrath of Krolm/Balance of Twilight bonus scenario, or an additional scenario?

EDIT: Nevermind, I found the demo. Apparently is a new mission not included in the Gold version.

Oooh! A link please.

https://www.fileplanet.com/35185/30000/fileinfo/Majesty-Demo

I didn't installed the demo, but extracting the contents I see a single mission named "Vampiric Revenge", and indeed there is no mission named like this in the full game... BUT...

when I tried to copy the file "Vampiric_Revenge.q" to the full game I found out that there is already a file with the same name !!
And it's a little different... I'm not sure how to play the mission, but it's already included in the full game... maybe some animations or sounds can be different from the demo anyway, because at the time the game was still a beta version...

With the last patch, the quest "Vampiric Revenge" is listed and playable in the "Downloadable Quests" :)

You're right!

The Balance of Twilight, the official downloadable mission for The Northern Expansion is no longer on the list but is still on the main map.