How Betelgeuse captured Subaru's body like in the ending of episode 23?
In episode 23 at 11:40.


   When Julius was having a conversation with Subaru, he said that Subaru might have an affinity for spirits. Since Petelgeuse was a spirit himself, he could possess the fingers' body which had been cast spell on beforehand and even entities having affinity for spirits. According to Julius, Subaru definitely matches the case.