A prank should be funny and enjoyed by the person getting pranked.
OP.
Using things she's deeply afraid of to "prank" her is not going to be enjoyable by anyone except you.
You want "psychological horror"?
Have someone call her and your family pretending to be the police department and say that you passed in a vehicle accident.
Point a gun (fake or not) at her and pull the trigger.
Find some roadkill and put it in her mailbox, then car, then bed.
Have some friends stalk her at her work or school and then follow her home.
Lay on the floor with an empty bottle of sleeping pills and a note that says "I'm sorry".
The other idea I have is to gift her this stuffed toy I saw which was available in different sizes. Then replace it with a bigger one every few days. This sounds like smth she'd quickly catch onto tho.
My wife does this. We have several that we have acquired over the years. She has them all named and puts them out like they are members of the family. We'll have one playing the piano, one doing yardwork outside, one sitting at the kitchen table, etc.
Play with one of those spirit boards then fake a haunting like with the help of a speaker footsteps at night, humming/laughter when shes alone, flickering lights, tv turning on randomly, have her find a creepy kid's drawing, if you have dogs maybe briefly use a dog whistle from a safe distance maybe even from outside, yknow all that paranormal activity stuff. Have fun but dont go to far lmao
She isn't a believer in the supernatural and will see right through the spirit board, but doing the same things without a board sounds promising. No dog unfortunately.
One year my wife got me with a random prank a few days before Halloween. I told her that it's OK Halloween is coming, and I'll get my revenge.
It turned out that she spent the few days in between checking everything and everywhere in the house. Every time she opened a door she did it carefully, in case anything jumped out or fell on her, she wouldn't let me make food or drinks in case I did something to them.
By Halloween, she was terrified, but didn't know what to be scared of. Her car was parked in the garage, and it took her about ten minutes to go into the garage, check around the car, get in, then open the garage door.
The day after Halloween, she asked me what the prank was because she must have missed it.
I'd completely forgotten about getting revenge five minutes after I'd said it to her...
Slightly adjust the brightness of your lightbulbs every day and act like you have no idea what she's talking about. Convince her that it's all in her head.
I love the pranks from Just for Laughs because the prankee always ends up laughing after finding out it's a prank. Here's a compilation of some Halloween ones they've done.
Of course, some of these are not practical for you but hopefully they'll give you an idea of a fun prank.
What types of pranks have you played on eachother? I think that would be helpful for gauging her sense of humor!
Harmless stuff that won't bring bad blood would be things like putting plain yogurt into a sour cream container and have her see you eat from it straight.
Oh if she has light hair definitely don't do any of thr shower food dye stuff cuz it might temporarily change the color and she will have to tell every coworker and person that asks what you did.