Marathon 2: Durandal

Marathon 2: Durandal is the sequel to the first game. Following the events of the Pfhor aliens have been defeated and the A.I. known as Durandral has been freed from it's containment. 17 years have passed and Durandral has assembled his own army from the colonists of Tau Ceti, including the Security Officer. Traveling across the galaxy, Durandral finds the home world of the S'pht, a Pfhor slave race, who Durandal liberates and who now worship Durandral as their Messiah. Durandal is searching for the location of a legendary lost tribe of S'pht who may hold the key to his mad quest for... well, whatever he's madly questing for. Unfortunately, the Pfhor dislike Durandral's presence and begin attacking his army.

Although the engine is similar to Doom in most respects (2.5D maps, bitmap sprites instead of polygons, etc.), Marathon 2 distinguishes itself with such features such as: a rudimentary inventory system, ambient sound, multiple fire modes for each weapon, allies who aid the player, enemies belonging to different factions which will attack each other, a more developed plot (gradually narrated to the player at various computer terminals scattered throughout the levels (much like System Shock's e-mail transmissions)).