It only happened once to me in my life.
Leading up to it, I threw off my sleep schedule massively going days without sleep, sleeping for a few hours in the middle of a day, and sleeping again in the middle of the day some time later.
I was in a sort of hybrid dream-awake state but definitely awake state. I was sleeping on my side and I could sort of move one of my arms arm, but only the wrist, and barely.
The dream fuzziness impeded this urge to check turn my head to look behind myself, but I couldn't, I could only get in wrist movements, formulating the sort of command to move my arm wasn't going through in my mind right.
I was the urge got worse very quickly, and my body was not responding and I kept fighting it harder and harder which I think made it really worse.
I tried yelling or talking but I could only breath or make gasps. It felt like having a locked up voice from a cold or something similar, and putting together a voice just wasn't doable somehow.
It was something very very very bad, there was a knife of some sort. There was a sort of increasingly terrifying aura about the room, like being stalked.
There was still a logical part of me saying that this was merely sleep paralysis, but I wasn't sure of what exactly it was, and I wasn't SURE, that there wasn't something or someone trying to kill me in the room, not being able to see the other half behind me.
I tried everything possible and ran out of energy, I just concluded that I was likely going to die right there and then and gave up. I woke up and was fully functional without any issues at all later.
I've been in a situation where someone tried to kill me, it was basically the exact same feeling, if not, even worse.
For a few weeks I became pretty paranoid about it happening again and I was confused on what to do for sleep, but I haven't thought about it for months now. I can't imagine what it would be like though for someone who regularly suffers from this sort of stuff, there isn't enough you can do in that state to be certain that you are safe.