This is the first official expansion module of the Neverwinter Nights series. In most ways, it works exactly like the original campaign, and this review will be covering the changes from the first game. The review of the first game gives details for the interface and gameplay.
If you have bought Neverwinter Nights Gold or Neverwinter Nights Platinum, this will be part of the pack. If you only have the original Neverwinter Nights or none at all, buy the Neverwinter Nights Platinum and have all the official expansions in one place.
The original Neverwinter Nights has very little control over your hired henchman. This game improves that situation in that:
This game introduced the prestige classes of arcane archer, assassin, blackguard, harper scout and shadowdancer. Each of these classes have some cool new special abilities making them worthy classes to aspire to. You won't be able to get these classes without careful planning as you advance in levels so watch the feats and abilities as you level up.
Another change in the game is that your actions can influence your lawful vs chaotic alignment. Are you looting chests in the town hall? That's another couple of chaotic points for you then. Other actions push you toward lawful. You can also go toward good or evil, depending.
Other game improvements are that it saves when you are about to hit a danger zone, and has some nice cut-scene animations. Some of the things you have to do require solving puzzles and actually thinking as opposed to just muscling through the game.
You can import your hero from the first game. If you have finished that game, then your hero will automatically be on the list of available predefined characters. However, if you do, this game will be a cakewalk. You should have finished that game at about levels 16-18, and very little until the end of the game will even vaguely match your power. Shadows of Undrentide is meant for a brand new character, not a continuation of your previous game.
4 1/2 stars of 5, not quite a perfect experience, but a truly excellent one.
You probably should create an account on the Neverwinter Nights forum site. It's good to have a forum for asking questions and getting advice. I also downloaded the hints file from UHS. There are so many things to do, it's good to have some hints.