One of the most important aspects of a story is the ending.  Most of the advice you hear as writers concerns the beginning of the story.  The Hook.

The beginning is important, yes, you have to hook those readers or they'll pass you by but the ending is rarely mentioned.  And yet the ending is as important as the beginning.  If handled badly, it can leave your reader thinking anything from 'Meh' to 'Well, that was a total waste of time'.

The opening of the story hooks your reader into reading that one story. If you want readers to look out for your name on future stories, you really need to give them a good ending.  One they will remember.  Ideally, one they'll lose sleep over.

Some bad endings are obvious, but sometimes they are only glaringly obvious after someone else points them out to you.  If you have a human protagonist helped by a supernatural agency (ghost, demon or such like), then letting the supernatural agency take over in the final moments is cheating.  The human has been battling all through the story and the reader expects the human to win or lose at the end by their own hand.  If their supernatural helper steps in at the last moment then the reader is left to wonder why they didn't just sort things out right from the start.  The human, the one they've connected with, wasn't needed after all so the whole story was pointless.  I made this mistake in my first attempt at a novel.  Fortunately, someone pointed it out in an early draft.  It's fixed now.  Must get around to submitting it again.

There are scenarios in which the power of the supernatural can take over legitimately.  If the companion ghost/demon was unable to use their power because of some constraint put upon them, and only the human protagonist could lift that constraint, then it can work.  I've used this one too, in another drafted novel.  It was complicated, but it worked out in the end.

In any such case, having the human part of the pair coming out unscathed is simply not realistic.  If you get involved with demons and ghosts you are very likely to get damaged, either physically or mentally. Probably both.  It's a horror story.  Nobody gets out without some pain. Develop and change that character using the terrors they experience.

In many horror stories the protagonist is dead by the end.  This poses a particular problem in first-person storytelling because if the person telling the story is dead at the end, how can you be reading it?  The protagonist did not survive to tell the tale.

If your main character is going to die or is indeed already dead, don't have them tell it in first person.  If you want them telling the tale through a medium, or by writing in the dust, or by carving it into someone's flesh, then make the medium, the dust-reader or the bleeding book the first-person teller.  You can frame a dead-protagonist story in such a way but if you're going to have a first-person corpse, there's no way to tell the story and make it believable.

Even the famous fall into this trap.  James Herbert's novel Nobody True started with the brilliant line 'I wasn't there when I died' but the storyteller is dead, so who is he telling his story to?  It turns out he's been telling it to another ghost but that other ghost still can't tell the rest of us so the story exists in limbo.  It's a step away from the first-person story written by the victim who dies but it is still a damaging ending.  It breaks the spell.  We can't have known about this story because it's confined to the dead.  This particular story also had an irrelevant reveal at the end, which further damaged it for me.

Once you break the believability of a horror story, it's just not scary any more and if it's not scary it's not horror.

If you break it right at the end with help from the ghost or demon who has done little or nothing of use in the rest of the story, or by revealing your first-person character as a corpse and therefore unlikely to be very good at holding either a pen or a conversation, your reader will not remember your story (and your name) as a source of fear and dread.  They will remember both as a waste of their time. That's not good if you plan to get more than one story published.

There are easy ways to avoid these traps.

If your main character is to die or is already dead, either write in third person or have a surviving character narrate in first person.  That sounds complicated and it can be, but think Sherlock Holmes.  All those books were written as if they were Dr. Watson's journals.  Holmes was the main character but Watson was the first-person narrator.  It can be done, and if done well it can be timeless.  Holmes didn't die in the stories but he could have.  The story would still have worked because Watson, not Holmes, was narrating.  It would also have continued to work if Watson was in contact with the ghost of Holmes—as long as Watson did the first-person part.

Watch for the too-easy escape at the end.  If you have trouble seeing it, join a critique circle and put your ego aside.  Pay attention to the readers who critique but don't be bound by them.  Sometimes they're wrong.  The trick is to know when they're right.  Most of all, the trick is to never be offended by someone's comments.  That's easy.

There are too many issues with endings to deal with in one short article. Another is the 'obvious sequel' ending, a prime example of which is the first Resident Evil film.  The ending of the film is obviously not the end of the story and that's not fair.

Unless your readers/viewers know from the outset that they are facing a series, they expect a complete story.  If it doesn't end they will be irritated.  As I was irritated at the fourth Alien film, in which Ripley, who had died in the third film, was reincarnated.  Now that is forcing it.

I'm not a big fan of Harry Potter (although I did very much like the Dementors as well as the murderous chess game) but those stories are a good example of a series.  Each story is complete in itself and each book begins with the arrival at school at the start of term, and ends with everyone leaving at the end of term.  That's a neat way to close off the story and still leave the next one open.

The Sherlock Holmes stories kept the same characters with different investigations.  A series should be a set of complete, individual stories, not a story with no end.  I mean, how many times can Michael Myers or Jason Voorhees die?  Once is enough for most people, even the very nasty ones.  Don't stretch credibility too far.

It's true that you need a good opening to get people reading.  I would argue that it's just as important to have a great ending so they'll look out for the next story you write.  You can have the greatest opening line in the world but if your ending is flat, nobody will read another one. If you're going to get anywhere, those readers have to want to see the next one.

Make sure your opening is great.  Then make sure your ending is great. All you have to do then is join the great opening to the great ending with a middle bit. Make that great too, and you're in business.

Who said writing was hard?