Exetior was just walking around the Blood temple. He watched as Amy, Sally and Cream talking and he did check up on Tails, Knuckles and Robotnik as well as Negagen earlier. This would save Mobius. His demonization plan worked however he felt something was off but ignored it as the Highests had no way of finding out about his plan. But he heard some footsteps and saw Sonic? Impossible...he was possessing him.

Exetior: "Sonic? How are you here while I am in control of your body. I swore I broke your spirit. I'm doing this to save Mobius so it had to be done. No hard feelings for it. But I control your body now."