There have been many names given to that dark entity over the years, but the most common, or at least the one I know, is the Dark Visitor.
Dark Visitor

Even when you don't know it he is there waiting for the right moment....

That entity is supposed to have existed since the dawn of time and is considered neither evil or good, but dangerous if allowed to be. It can take many forms, but at most times when it appears to humans, it takes the form of a tall, slim, human-like dark creature.

It is said that it searches for weak people, people who are sad, have lack of motivation, or simply can't stand on their own. When it finds somebody weak it sticks to him, feeding on his very own vital energy until the victim eventually loses all hope for life and dies in a mysterious way or commits suicide. Victims of the Dark Visitor become antisocial, constantly depressed and exhausted.

The Dark Visitor chooses to feed when a person is sleeping, especially at night, as we are most vulnerable and he is most powerful. If you have any of the symptoms and the Dark Visitor is already on you, you can see him simply by attempting astral projection while you sleep and he will be there. All you have to do is strongly imagine him before you sleep and try to visualise him in your room. Trust me, he will appear if he is on you already.

If you attempt to see him and he realises it, he will suck your entire vital energy and you will suffer a painful and slow death. As time is different in our sleep, he can kill you in seconds and make it seem like months or years of pain.

There is also a way to summon him, but I strongly recommend you don't do this, as it will possibly result in your death. The only way you can see him is in your sleep and you will need the following things to do the summoning:

  • 8 white candles
  • A photograph of you being happy (it doesn't matter what you do, it just needs to show you being happy)
  • A drop of your blood 
  • A mirror which you can move and that can depict at least half of your body
  • A piece of paper and a marker
  • A lighter

The summoning must take place after midnight. You start by lighting the candles and placing them around your bed in a circle. If your bed is very close to a wall do that in the part around your bed. Then place the mirror somewhere near the bed where you can see yourself while being in your bed. In front of the mirror place the photo of you being happy and drop a little bit of your blood on the photo. The Dark Visitor will use the mirror as a portal, as he is mostly a being that travels in between many dimensions and the mirror will make it easier for him to appear. As for the photo, he is drawn to anything that reminds you of happy moments, as he seeks them to suck the positive energy you have from them, and the blood will attract him even more to you. Then take the piece of paper and write the following words with the marker:

"Martenmus bisas mande kokepati vandersal"

There is no known translation to this, it is just rumored to be the words that summon him in an unknown language. You will then burn the paper, and place what is left under your bed. This part can be skipped, but know that doing this will not ensure that he will appear. If you don't do it, there is a slight possibility he won't come, as these words call him in a language only he understands. 

Now all you have to do is sleep in your bed. As soon as you fall asleep you will still be in your room in a dream-like situation, but guess what? He will be there. After he sees you he will slowly come close to you and try to suck all your energy. The good news is that you can try to walk away from him- leave your house and run. At dawn, you will wake up if you manage to avoid him, but know this: the time he will be chasing you may seem like a month or even a year in this dream-like situation, so it's close to impossible to lose him. And even if you manage to wake up and survive the night he will still be following you and sucking your life energy slowly every day, as he does to his random victims. But because you summoned him, you will die faster than a normal victim.