This guide to the Shades of Darkness, an expansion of the basic version of Might and Magic: Heroes VI contains:

thorough walkthrough of the Dungeon campaign and the second Undead campaign

description of the side missions

a list of the Dungeon units, together with their use on the battlefield

The walkthrough of both campaigns assumes that you're using the default hero. In case of the Dungeon it's Raelag (Might), while for the Necropolis it's Vein (Might). All missions are balanced with those two in mind, therefore playing with a Magic-oriented hero might be much more difficult and require a different approach to the gameplay.