Neverwinter Nights (NWN) is a computer game set in a huge medieval fantasy world of Dungeons and Dragons. This role-playing game (RPG) puts you at the center of an epic tale of faith, war, and betrayal.
Played it only recently, it had been put aside (totally!) for a couple of years without even finishing Chapter One (and barely starting it).
It's quite fascinating and long, and immersing due to an intricate plot and many awe-inspiring side quests.