It has Obsidian's best story since Planescape: Torment, and it provides a very unique role-playing experience.
In some ways, Mask of the Betrayer feels like the game that Neverwinter Nights 2 should have been. Thanks also to extensive patching, the interface and controls now feel like they should have been built from the beginning. The legacy issues carried over from the original Neverwinter Nights are finally gone. You can now pan the camera somewhat away from your characters and use click-and-drag to select multiple characters. These features are not introduced by the expansion pack, but they make it more worth revisiting. The music has been substantially improved for Mask of the Betrayer. The soundtrack is brand new, and so are all of the spell chants. Neverwinter Nights 2 reused too much of the audio that first came out in 2002. This game finally does away with them. The soundtrack is a very nice surprise – it is somewhat low key and perfectly fitting for the story and setting. It does away with the generic, bombastic RPG score in favor of a more unique sound that constantly reinforces the story's dark tone. Like every other Obsidian game, the voice acting is also top-notch.
The centerpiece of Mask of the Betrayer is its story. It is excellent, and one of the most unique stories in years. Instead of providing the same old, same old, "Save the world from a great evil" narrative that is in 90% of RPGs, it gives you a more personal quest. Your ultimate goal in the game is to rid yourself of a horrible curse. The personal nature of your quest is reminiscent of the story in Planescape: Torment, which was also unique. As with Planescape: Torment, your condition dramatically affects the world around you, so while it is important to you personally, it has far reaching effects to everyone else. Also reminiscent of "Planescape" is the plane-hopping nature of your journey. Along the way, a small number of memorable NPCs can join you. Unlike the NPCs in Kotor 2 and the original campaign, they aren't horribly dull or full of pretentious dialog. Their back stories are interesting, and they fit into the main quest very well. The quests and the areas that you visit show a lot of creativity. The influence system finally works and adds something to the game, after a couple of fumbles in Obsidian's previous efforts.
For the most part, Mask of the Betrayer is not a combat-heavy expansion pack. Dungeons are typically pretty small and only have one level. Battles are fewer and further between than they were in the original campaign. This is a welcome change that allows the game to focus on its strength – the story and dialog.
The most interesting, and most controversial, addition to this game is the nature of your curse – the spirit-eating mechanic. In Mask of the Betrayer, you must consume souls to survive, as if it were a drug addiction. If you go too long, without consuming a soul, you lose health and power. If you consume souls haphazardly, your craving gets worse, ala a heroin addict who needs a bigger and bigger hit to get a fix. Anyone that you meet who knows about your condition reacts to you negatively. It requires a lot of management and minimizing activities like travel and rest. Contrary to some of the extremely negative views that you may have read, it isn't terribly hard to keep under control. It is an inconvenience, but not a terrible one. It serves its function of giving you a strong motivation to try and get rid of it.
Mask of the Betrayer is not without its issues. Some of these problems are related to the use of the Dungeons and Dragons role-playing system. This system is best when characters are somewhat lower level, but it gets messy at the higher levels, and in this game, your characters get to a very high level. The real-time-with-pause combat system has always been a non-ideal compromise when it comes to D&D combat, and it gets worse as the pace of the battles picks up. Battles sometimes whiz by in an indecipherable blur. Spells go off in bunches and melee specialists launch flurries of attacks that can drain a well-armored character of 150 hit points in mere seconds. Sometimes, numerous spell effects are stacked upon one another in an area, so that it is almost impossible to see what is going on in a battle. You can read what is taking place in the text box at the bottom of the screen, but you have to scroll way up the page just to find out something that happened five seconds ago.
This expansion pack could have also used a lot more playtesting and balancing. The difficulty curve is wildly uneven. One moment, you are slaughtering fodder, plowing through 35 XP enemies by the half-dozen. The next moment, you are getting brutally ripped apart, reloading your saved game repeatedly you finally cheese your way through a nearly impossible battle. The game's final area has a series of laughers, followed by the impossibly hard final boss. The chaotic nature of the battles makes it difficult to figure out what you should have been done differently. It is an occasionally frustrating affair that makes combat more of a liability to the game than an asset.
Mask of the Betrayer is not always the most accessible game, but between its unique story and epic levels, it has a lot to offer role-playing fans who are looking for a quality story-driven experience. If you are fond of Obsidian's storytelling talents, and you can tolerate a few flaws, you should be very pleased by this offering, which is ultimately the best yet in the Neverwinter Nights series.