The manual for The Witcher 2 lists the various difficulty level, most of which seems to be pretty standard. The last one did catch my eye though:
Dark – for very experienced gamers. This difficulty level allows you to acquire dark
items and grants you access to quests which are not available on other difficulties.
Although it is very unlikely that I'll actually be able to play at this difficulty level, I am rather curious about what these quests and items are, and how they affect the story (use spoiler markup as necessary).
Best Answer
Calling them new missions is really a bit much. Basically, Dark Mode adds three new sets of equipment - one in each act - consisting of Armor, Boots, Gloves, Pants, and a Steel and Silver Sword. These items must be crafted - the recipes are purchased from a vendor at a very high price, and the materials lists are staggering - and are 'cursed' - the swords will do damage to Geralt when used unless the full set of matched equipment is worn.
The 'quest' aspect comes from a token journal entry added with each set which weaves together a legend associated with the three sets of gear.
All of the information about the Dark Mode sets, including materials requirements, stats, crafting information and bonuses can be found on the Wiki