This one’s been floating around for a while. Makes sense, given that emotions are magic in this world.
 
My guess would be that the gases released from ponies when they experience negative emotions (see “magic is gas” theory) join some giant sapient cloud/atmosphere of magic around the planet. This cloud tries to overpower creatures and brainwash them…
 
The weaker the pony affected by this, the worse it gets (the less the imprint of their personality remains - yes, I got the ‘increased/increased amount of imprint’ the wrong way around in the actual image) - otherwise the ponies can fight it off to some degree. Stygian is unable to fight if off, so it basically reveals its true form - Sombra retains its cruelty and hate, while with Nightmare Moon, all that remains is the desire to dominate overlaid by Luna’s pride and desire for attention.