Operation Templar
From FreeSpace Wiki
Operation Templar was a series of missions in which the Hammer of Light was crushed. These missions were to be played in the multiplayer co-op option in Freespace 2 originally.
The Game of the Year edition includes an unmodified, unbalanced single-player version; if you want to play this campaign at it's intended difficulty level, start it as a network game with yourself as the only player, so you will get respawns.