I had this recurring dream when I was a kid. It would happen when I was sick and my fever was high. I remember it was terrifying. All the more so for its indescribably abstract nature. It was a dream of ideas. Crushing wrong ideas that I could not pinpoint and was punished for trying.
I'd awaken in a sweat, petrified. Occasionally screaming. And my mom would come. Or my dad. That calming looming shadow beside my bed ready to help. Patient, kind, nurturing. But the panic of fever induced nightmares stays above the bed, waiting for you to drift back so it can tackle you again and I knew. I needed to explain for them to help me.
The dream defies explanation. It is a substance of indeterminate size. Just when you think it is a large black, velvety antimatter coming to claim you, it shrinks. It is thin when you say it is thick, long where you think short. The moment you commit to its shape or size, it makes you a fool and destroys the thick place within yourself you didn't know you had. When you call it tiny, it takes a humiliating bite out of the tiniest value you have left. It leaves you panting as it laughs, then humiliates you for saying it had a voice in the first place.
I've had other repeat dreams since that one. Snakes, teeth falling out. But nothing is scarier than something you can't even describe. That dream was fear extracted and abstracted. Then it humiliated and multiplied.