Fear.
We think we know what it is until we experience it.
Then, and only then, do we recognize that fear is a physical being, an entity with weight and force, writhing with malice. It is a monstrous—yet ephemeral—demon, a translucent organism comprising rage and loathing that fuel its ravenous thirst for vengeance, leaving dread in its wake.
The only weapon sufficient to conquer fear is acquiescence. By yielding to fear, accepting it has vanquished bravery, an even more fierce form of fear can arise, the fear of losing the last shred of humanity to a monstrosity. Then, and only then, do we recognize we have the capacity to overcome our worst nightmares.