Arx: End of Sun - a standalone Doom 3 mod - is a prequel for Arx Fatalis:
More information.Arx - End Of Sun is a new free and fan made game that is based on the first person dungeon RPG: Arx Fatalis developed by Arkane Studios. In our game world you will play out the events ( with a liberal dose of poetic licence ) that precede the original story of Arx Fatalis. You will no longer require a copy of Doom3 to play Arx - End Of Sun. We are now running on a custom version of the engine code and we use no assets from any commercial titles in our game.