Nightmares were formed from negative emotions—jealousy, bloodlust, malice.
Of course, those were emotions born within nightmares. Real-world emotions were not under their jurisdiction. In the Material World, there was the Reservoir of Light formed by emotions, and that energy had been claimed by the various Lantern Corps.
So when the others began accusing it, the Nightmare that had become the Batman Who Laughs flew into a rage from shame. It raised a hand and gave the elderly Joker in the wheelchair beside it a hard slap.
"Smack!"
"Why did you hit me? I wasn't blaming you!" the old Joker complained, clutching his face.
"Because I don't like the look of you." The Imposter Batman Who Laughs was really picking on the weak, using violence to establish its authority among its kind. "Look at me again, and I'll eat you!"
Normally, Nightmares did not communicate like this, much less argue with each other during a mission, lest Insomnia settle the score afterward.
But this was Harley's dream, and she still retained some control over this place.
For example, she could subtly alter the enemies' behavior. The influence of language had always been mutual. Nightmares could influence people, so people could naturally influence Nightmares too, turning them into slapstick performers for her amusement.
"A Batman still dares bully people? Did you all see that?" Harley shouted from not far away, hopping in place. Her head kept shaking in vigorous disapproval. "If I were you, I wouldn't stand for it! What are you, a bunch of cuckolds? You're all Jokers! What should you do when Batman hits you?"
After receiving that little bit of "outside guidance," the many Nightmares that had turned into Jokers suddenly understood.
That was right. They were playing Jokers now, so they had to act like Jokers.
"Die! Hoho!