Star War: Knights of the Old Republic II - The Sith Lords (KoTOR2) is a PC/Xbox CRPG game originally released in 2005/2006, developed by Obsidian Entertainment and published by Lucas Arts.
It takes place in the Star Wars universe, and is a sequel to the original Knights of the Old Republic game. It uses an adaptation of the popular D20 rules system, and includes several mini-games throughout the game itself.
The game has extensive dialogue and a large variety of quests. Some characters you encounter will join you and aid you in your quest, with two of them able to accompany you most of the time. Your interaction with them and others will determine how much influence you have with them, and how they react to you.
As with KoTOR, your actions determine your alignment - you will find some characters react better to light and some to dark side actions.
The game is set 40000 years before the Star Wars movies and five years after the events of the original game. The Jedi Order has nearly been destroyed in a brutal civil war, and the Republic is on the verge of collapse. The Sith have spread throughout the galaxy to hunt down the last of the Jedi.
After a short tutorial section, your character wakes up in the medical facility of an apparently abandoned asteroid mining station.
In the rest of the game, you travel through the galaxy seeking out the surviving Jedi, and a means to defeat the Sith Lords.
Various fan created tools can be used to mod the game, but some can be made with readily accessible graphics and text programs. Check the Holowan Labs link for information on modding.