Complex sentence: At the same time, the world is being afflicted by hordes of monsters created by Dr. Faust, the game's main antagonist and a mad scientist was responsible for the deaths of Gretchen and her fellow witches.
Split into simpler sentences: At the same time, Dr. Faust, the game's main antagonist, creates hordes of monsters that terrorize the world. A mad scientist is responsible for the deaths of Gretchen and her fellow witches.