Complex sentence: The Greek Underworld was the one major mythical location that the developers of "Titan Quest" had not yet approached, offering offered a rich source of material for their creative staff, so it was made the location for the expansion.
Split into simpler sentences: The Greek Underworld was the one major mythical location that the developers of "Titan Quest" had not yet approached. As it offered a rich source of material for their creative staff, it was made the location for the expansion.