This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Insidious
The idea of being hunted by body-snatching spirits in your sleep/astral travel, especially if you've had dreams resembling Astral Projection yourself. Thankfully, in real life, if something goes wrong, waking up is usually as simple as just desiring to be out of the nightmare. The characters encountered also can't interact with the waking world or obstruct your ability to wake up...usually.
The claws clicking at the end of Chapter 2. You didn't even need to see him for your stomach to jump to your throat. Elise's look of horror was all you needed.
And at the end of Chapter 3, he makes his official onscreen return.
The old lady is creepy.
And said 'old lady' is actually the ghost of a male serial killer named Parker Crane. And his back story doesn't help; he was a transvestite who dressed as an old woman in a black wedding gown whilst committing his murders, and after being hospitalized for self castration, he committed suicide. It's all because of his cruel, abusive and downright terrifying mother who beat him into believing he was a girl. Seriously, she (a regular ghost) is arguably more terrifying than the demon!
To add, the way he had...well...the assemblage of his victims corpses when found by Loraine, Carl, Specs and Tucker in his home. It becomes much more unsettling when they make an appearance in The Further version of the Crane home, when Mrs. Crane hides among them.
During the second film, Renai receives a phone call from the detective working on Elise's murder, informing her that no evidence of Josh's DNA was found on Elise's neck marks. This means that Parker Crane's influence is so powerful he's able to change his host's DNA.
Quinn getting thrown from her bed and injuring her neck, meaning that all she can do is lie still and watch as a demon walks around her room, turning off all the lights and getting ready to attack her.
Demon-possessed Quinn effortlessly taking down three other people, then breaking away her casts and walking towards her father on broken legs, complete with horrible crunching sounds.
The faceless, limbless representation of Quinn in the Further.