There are two things that possess people: Demons and spirits. 1. Maybe it's Vergil. They seem to want us to think it's him, but I don't think Vergil's dead. 2. It might just be a new demon character, which would be very cool. 3. Or, it could be Arkham. He lusted for power, remember? And even though he was a human, he was able to attain great demonic power. While he wasn't more powerful than either Dante or Vergil, he was able to push them around a little.