My daughter had them when she was little. As you say, pretty scary stuff. She would sit up in bed, eyes wide open, watching something in the room but she couldn't see me or her mum. She would be in utter terror. All we could do was hug her and tell her she was safe. Eventually she would come back to her senses and then, weirdly, she would always need a very long pee!! Made us wonder if there was any connection.
She never remembered a thing next morning either. She also had imaginary friends and sometimes heard voices too which was a bit spooky.
Out of my three kids she was the only one to go through it. We had about two years of it on and off but she eventually grew out of it luckily.
I know it is worrying and disturbing but just stick with it, it will pass, it is pretty common.