Read The Bridge
 
Rarity felt herself lapse back into the waking world, the gentle chills of Luna’s magic trickling off her like she was stepping out of a bath. Yawning momentarily and taking a stretch from the first relaxing sleep she’d had in days, brief as it was, Rarity didn’t need to ask to be given her diagnosis.
 
“There are definitely still trace amounts of the Nightmare in your consciousness and magic.”
 
Luna muttered with shame in her tone. Her horn was still glowing as lenses of magic had condensed over her eyes like glasses. The alicorn emitted a pulse of magic across the room and in a second’s lifespan the same sort of lenses had formed over Rarity’s eyes. The aura visible to Luna became visible to her too and the unicorn immediately followed Luna’s line of sight to gaze upon herself. Aura revealed the truth of one’s self. Despite having originally been made to counter changeling disguises, it had also proven useful to detecting the nature of one’s being. In the case of weeks earlier, revealing the true nature of Canterlot’s six most recent guests. In this case, Rarity could see a veil of royal purple identical to the Element of Generosity across her form, hugging it like a flickering aurora. But here and there were blotches of blackness and dark purple that stained her personal aura, so dark they looked more like holes in space than they did coloration. Understandably, Rarity was struck with a jolt of terror, instinctively swiping at the darkness to try and force it off her. She was about to cry out when a hoof gently held her mouth shut. Rarity quivered, stifling tears and closing her eyes. The Princess of the Night’s voice quietly hugged her.
 
“Contrary to what some believed, the Nightmare is not solely some outside force that possessed us and is entirely responsible for the end result. Rather, it’s a manifestation of what dark potential is already there, bringing out the thoughts of that individual that are usually suppressed or tempered and not acted upon. To bond, it requires something already there and a willing host.
 
The Element of Generosity looked up to the source and saw Luna, aura and all. In her case the aura emanating from the alicorn was positively blazing, no doubt tied to her vast magical potential as a near peak individual of all three races. Largely radiant blue in color, sparks of auburn, sky blue, and neon red that indicated her former Elements. And just like Rarity, the aura coming forth from her predecessor in Nightmare was similarly scarred with dark blotches that mired her appearance like some parody of somepony spattered with paint. Rarity looked back at her own dark patches.
 
“S-So, I shall always be prone to such horrific dreams and reflections because deep down, I-I am some monster out of Nightmare Night?!”  
She sobbed. Luna sighed, tilting the unicorn’s head back up. Her face was a mishmash of concern and grief.
 
“Juliet Rarity Belle, listen to my words. Any sentient being with the power of reason will have the potential for good or evil thoughts, words, and deeds. The Nightmare is an expression of the latter. Had that darkness been fully expressed and let loose in anypony, then anypony from my sister to the Bearer of Kindness could have the potential to become a Nightmare.”  
Luna spoke, looking to herself and pointing to her aura’s blemishes.  
“Guilt, shame, envy, anger-”  
She punctuated each statement by pointing to a black blotch.  
“-it’s in all of us. Some more than others.”
 
Luna shrugged and lifted her wings to fan them out, revealing even more blotches. Luna didn’t need to look to know how many she had, she’d known for a long time. There was good reason why her views of Godzilla Junior had changed as much as when she saw his true self lying wounded in her bed as she did after the gyaos attack. She was looking at a physical reflection of her mental mires. Rarity’s eyes widened.
 
“Princess Luna you’re-!”
 
Luna cut her off, raising a hoof and shaking her head slowly.  
“Getting better, fear not for your princess in this regard…”
 
She lowered her hoof and sighed.