I cant complete "The hunt begins"

https://youtu.be/CBsYo6hdLAY
I can't do the quest, most likely it's broken or I don't understand something, the video shows it. it says I have to talk to him, but I can't do it, and when I follow the sphere and it gets possessed and I kill it, nothing happens.
help me, because i can't play the quest anymore because of this quest