Neverwinter Nights 2: Mysteries of Westgate (NWN2:MoW) is an expansion pack for the computer role-playing game Neverwinter Nights 2. It was developed by Ossian Studios and published by Atari on April 29, 2009. The player creates a character and controls it, along with a group of three pre-designed companions, journeying through the game world. The gameplay is very similar to that of the base game. Mysteries of Westgate also includes new monsters, music, and other tools, which can be used by players to create their own Neverwinter Nights 2 levels.
The game takes place in the Forgotten Realms world, a Dungeons & Dragons campaign setting, in the area of Westgate. The player creates a character at the start of the game who finds a cursed mask belonging to the "Night Masks", a thieves' guild at war with two other organizations. The player chooses which of these other organizations to side with, and embarks on a quest to lift the mask's curse.
Source: Wikipedia, "Neverwinter Nights 2: Mysteries of Westgate," available under the CC-BY-SA License.