Question Mark Pt 3 is the twenty first story arc in EGS:NP. It started with EGS:NP Comic for 2015-04-08 and ended with EGS:NP Comic for 2015-05-18. It's canon and follows after the first part, although not immediately: it started at night with Sarah trying to sleep and not having much success. Then Pandora Chaos Raven appeared before her (as small girl, floating, with light) and saying that she almost didn't marked her and that she can't even really watch what Sarah is doing with her spell.
Sarah reacted with sass, saying that Pandora should warned her before making light. Pandora wonders about her reaction to ancient being of tremendous power, and Sarah explains that she reacts to paranormal stuff in one of two ways: curiosity or sass. Pandora hurt her eyes, so she get sass. Pandora replies that she made good decision with Sarah.
She then explains the less apparent part of the spell, starting with how dangerous it is: not directly, but because she can discover things people kill over. She specifically warns her to not tell Edward Verres, mentioning that he would be likely to add that to her file in DGB and that the last person with this spell was strong-armed into government work.
Then she talks about how Sarah can abuse the spell for frivolous fun: with enough practice, she can move in the simulation ignoring laws of physics, make whatever changes she likes and even make the people act out in illusion of autonomy. The disadvantage is that the more she changes, the less accurate the simulation becomes - her imagination and expectation starts to affects even things she didn't wanted to change. For completely random example, when checking for magic marks, finger snapping their clothes away could result in seeing mark that doesn't truly exist, while manually undressing them or switching bodies with them would be more accurate.
Sarah's reaction to switching bodies told Pandora that she switched body with her sister. Sarah responded that that part of the spell is on a hair trigger.
The third point is that Sarah actually have affinity for this spell. Sarah mentioned that another immortal didn't found that, and Pandora explains that this affinity is hard to detect and that Sarah doesn't have enough power to use it, only being able to cast it inside the Moperville's high ambient magic area. And that if she wants to awaken and have magic after this anomaly disappear, or elsewhere in the world, she needs to use that spell a lot.
Pandora then mentioned that the original idea with the shape of mark was to troll Sarah, because she expected it will take longer for her to find out what it does. Sarah blushed realizing that Pandora saw her naked, and Pandora reacted that she's hundreds of years old so it's nothing new to her. Sarah remembered the 200 years limit and asks how old Pandora is, and Pandora answered little more specificly: that she is answering question to her age with 299 for a few centuries now.
She then mentioned that other immortals may also be observing Sarah and her friends, and that as long as they abide by certain agreements, immortals are only seen when they wish to be seen, and that includes other immortals.
Pandora finished by saying she now wants something in return from Sarah: to not be boring. Then disappeared with her light.
Sarah concluded that she will not be sleeping tonight.