At the beggining, Mark Protosevich was the person responsible to write the Mass Effect movie script. He contributed the screenplay for I Am Legend and Thor. However, a new report seems to suggest Legendary is starting from scratch once more. Variety reports assistant editor turned screenwriter Morgan Davis Foehl has been selected to take on the tricky task of translating the role-playing third person shooter game into a cohesive and exciting sci-fi action adventure. Each version of the Mass Effect has been a bestseller, which suggests this project would have a massive marketability right out the gate. However, fans of the franchise wonder how the game—the direction of which is shaped largely by its user—will translate into a film.
The first idea of the film was to tell the story about past of Commander Shepard.. The Skyllian Blitz. After they heard the negative comments about the idea, they decided to transform the game's plot into the movie.
Skyllian Blitz - Mass effect wiki
Mass Effect Movie