Bill Cipher is a demon who can manipulate spiritual consciousness and the rules of reality at will. Its image is a golden one-eyed isosceles triangle, but it can enter other people's bodies and seize consciousness.
Harry: I would rather trade this fame for my parents to live with me!
Bill: well well well Lightning, I can help you make it happen.
Harry: Really?
Bill: Take my hand and seal the deal.
Harry took the demon's olive branch.
Bill: Now - your body is mine!
Harry: What!
Harry's soul was forcibly pulled out of his body.
Bill: Sorry kid~ We've made a deal.