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Mask of the Betrayer feels more completed and finished than its predecessor which felt rushed, and has the intriguing setting of Rashemen. Spirits, Hags and Red Wizards are abound.
There's load of new content too: the plane touched Genasi. There are four different types, air, fire, earth and water. The water ones have particularly cool effects. There's the half-drow which are basically the same as half elves but have darkvision. Go figure. Wild elves are also another new race which have a slightly higher DEX. You also get 2 new base classes and some new prestige ones too. One in particular, Spirit Shaman can be really fun to play because of all the spirits within the setting.
For me, the most exciting part of the expansion was the new setting and the potential for replay due to more opportunities to role-play and greater player choice.
Mask of the Betrayer has some interesting companions, more in depth than in the original campaign and with much improved, more emotive voice acting. Rather than the almost travelling-zoo sized party and companions from the original campaign, the expansion has four main companions, although at one point you have to choose between one or the other. This will mainly depend on the type of character you choose to play, so it shouldn't be too much of a problem.
Ultimately the game is grittier, more involving and has some epic battles. The story really pulls you in, which is nice if you felt cheated by the OC's ending.
However one disadvantage is the spirit eater mechanism, it can really cripple gameplay if you don't keep an eye on it. Although there are cheats that can be used, or just by being careful and taking advantage of cut-scene chances to use it, which keeps craving down. I didn't have many problems with it my first play through but on my second it became annoying. Also, the camera has been improved but it can be still very limiting within dungeons and enclosed spaces.
Still, it looks great and it's really cheap!
Major improvements to the Neverwinter Nights franchise: enhanced graphics, including extensive use of specularity, environment effects, and normal maps, improved performance, streamlined party control.