Shaken after worst nightmare I've ever had

I'm new here. In fact, this will probably be the only post I ever make, but I need some help here. It's 2am and I am reeling after a pretty terrifying nightmare that felt absolutely real. I'm 28, married and have one kid. I'm not a religious person, but what I just experienced is making me second guess that.

A few years ago, I visited my grandfathers grave (i think it was the anniversary of his passing). We were very close and he passed in 2001. While I was there, I didn't speak but I did ask him to watch over my grandma (his surviving wife) and, if he could, to let us know that he is still here in some aspect. I asked this because we thought she was in the early stages of Alzheimers - confirmed in late 2015. That night, my mom had a dream. I didn't tell her that I had gone to my grandfathers grave or anything. She called the next day and explained that in her dream, my grandfather was watching over my grandma. I heard the words she was saying, but it didn't click. Later that night, when I went to bed, it hit me like a ton of bricks. I'm sure it's possible that it was just a coincidence or that it was just wishful thinking, or maybe a mixture of both. But the timing of it all made me break down and cry. My wife had never seen me cry before and I explained everything and she said that it's likely our strong relationship has more or less not ceased - that maybe it's possible loved ones do look over the people they leave behind. This made me question if all that stuff I had thought was... putting it bluntly, BS... might be worth considering.

I thought it was necessary to give that background on what I'm about to say about the nightmare I just had.

My wife and I put our daughter down to sleep at about 7. We went to bed at 9:30-10:00 and I woke up from this dream around midnight. So I wasn't sleeping for that long... Anyways, here goes... It was a very very short dream. I dreamt that my immediate family (wife, mom, cousins, uncles, ect) were at my grandmother house and we were sitting at the big round dinner table, trying to comfort her because she thought she was possessed. A few people tried to console her and what not and then I tried talking to her and she looked at me with the most evil look in her eyes, slammed her fists on the table (which shook the room, keep in mind she is a little old lady) and... how do I put this? She yelled, but it was not her. Think of the most evil, terrifying laugh/roar you can imagine. Everyone in the dream started to rush out of the room and right as I got up, I woke up from the dream. But it felt REAL. I'm a 28 year old veteran who considers himself very tough. But it was paralyzing. I sat there for a second, unable to move and contemplating if I should even open my eyes. I legitimately thought the devil himself might be there with me in our bedroom and it just shook me to the core. I woke my wife up because she is religious and I felt like I needed her to pray. It was so odd. At the expense of looking like a fool to my wife, I woke her up in the middle of the night and asked her to pray because I was so scared. I've been shaking and even cried a little bit for 2 hours. I even CALLED MY MOM lol... She is also religious and I just wanted to connect the dots with her... Like I said, I'm not religious, but it's all so weird. While I was on the phone with her, she prayed for the family and specifically my grandmother - keep in mind that she is struggling with Alzheimer's. My mom even called her to see if she was ok (she is).

I'm not sure if anyone will even respond to this, but I just want to see if anyone out there has any sort of understanding of what this might mean. I don't know if I am spiritually sensitive or what. Or if it's stress. I don't normally have dreams and I especially don't have nightmares. Even more rare is when I remember them and this one was LIKE I WAS THERE. It was so real. Terrifying... I'm sitting here with my grandfather's bible in front of me, unable to sleep.

I will revisit this and hope to see some interpretation of what it all means. Thank you for reading.

It's just a nightmare.
If it meant something you should know right of the bat. The mind usually links back to the source of whatever dream element you're thinking on, if it doesn't it's probably random content mixed up. only you can make sense of it, it there's sense to be found.

Anyway, nighmares are awesome entertrainment and most of all, they make you remember you're alive.

A good dream or an intense nightmare, to me they both reasons to being a good mood in the morning!

Yeah, just meditate, and stay calm. Dreams have the ability to exaggerate emotions and feelings, but this is because they're only in your mind. But as long as you stay calm and carry on, they can't hurt you, and they will only get better as you learn to control them.

Lucid dreaming is actually one of the best tools to deal with reoccurring nightmares.

It sounds like you might have engaged in the beginnings of sleep paralysis. I have it happen often, and many times I will be engaged in a somewhat regular dream when a person will change into something horrifying or a voice that sounds 'demonic, evil, disembodied' (only way I can describe it) speaks as if it is an active participant in the dream. After this full blown sleep paralysis is engaged and I cannot move and can hear a voice speak next to my ear (that is the perception at least).
I don't know if this is very helpful, but I do find it similar to the voice you described coming from your grandmother.