As the Corruption grew, Celestia was starting to develop feelings of suspicion towards Twilight. Whenever they would pass by, she would often see Luna and Pinkie with the former discussing about private matters concerning the Corruption. Celestia had no idea why, but they were talking about private matters that didn't concern her. She would often see Twilight and Pinkie come back with bottles containing some form of corrosion. Part of her didn't wish to believe it, but she believed that it was the same corruptive ooze from outside the Crystal Empire. It had to have been true, considering that she was never called in for many meetings. Having had enough of this, she summoned Starlight to her private quarters.

"Yes, Princess?" Starlight asked.

"Have you seen Twilight around?" Celestia asked.

"Not that I know of. Why do you ask, Your Highness?" Starlight asked.

Celestia went towards the door and opened it. She moved her head around to see if there were any eavesdroppers anywhere. After checking, she closed the door and turned towards Starlight.

"Whenever you were out and about, did you see Twilight with some form of container? Anywhere around her bag?" Celestia asked.

"I dunno. I guess it looked like she was hiding something suspicious. But I don't know what's in her bag. Although I may have felt some sort of ugliness in it." Starlight speculated.

"Starlight, there is something I want you to do which goes against everything that I stand for as a princess, but I need you to keep an eye on Twilight. I need you to see what is inside that bag and find out if there is something she's hiding. Once you do, I want you to report it to me immediately. No questions asked." Celestia ordered.

"But what if Twilight and Luna find out about this? Won't they-"

"I will see to that matter myself. I just need you to find out what's inside, so I can figure out what these two are up to. If it really is what I believe it to be, Equestria will be in even more danger than we can fathom."

Starlight had this worried look on her face like Celestia knew something. She couldn't figure out why she was chosen, but she knew better than to question the Sun Princess.

"You know what you must do." Celestia said.

"Of course, Your Majesty." Starlight responded as she bowed.

After she was given a list of instructions on what to do, Starlight left Celestia's room and started looking for Twilight and Pinkie. Moving around the Crystal Empire, she started searching for the two of them. Fifteen minutes later, she found them. They were walking towards the castle where Luna and Cadence were waiting for them. She had no clue as to why they wanted to see the two of them, but she kept moving. Keeping a short distance between them and herself, she started eavesdropping on their conversation. Listening closely on them, she was shocked to learn what was inside the bag. Twilight opened up the bag and showed it to Cadence and Luna. Upon seeing it, her suspicions were confirmed. It was a canister that contained the same corruptive ooze from outside the Crystal Empire.

"How could they..." Starlight thought before ducking her head back.

Having listened to their entire conversation for about a whole hour, the girls left the room to head elsewhere. After they left the premises, Starlight immediately left the throne room and headed to Celestia's quarters. She had no idea how the latter was going to take the news, but the former already knew she was going to be disgusted beyond belief. She entered Celestia's room where the alicorn was waiting to hear what happened.

"What happened? What were they discussing?" Celestia asked.

Not holding back anything, Starlight says,

"I overheard them talking about the various executions they performed on a multitude of creatures to see what they can do to bring ponies back. Twilight was even using some herself to give her more power."

"What?" Celestia asks.

"It's true! They-"

Just then, Celestia interrupts Starlight and says,

"Starlight, I need you to do this for me from now on. I need you to spy on Twilight more and uncover what she plans on doing. Then come to me and tell me everything on what happened. Okay?"

"But what if she-"

"She won't. You were her pupil. I'm sure you'll find a way." Celestia assured.

Looking towards Celestia's door, Starlight knew that she wasn't given a choice on whether or not she wanted to keep spying on Twilight and Pinkie. The very last thing that she wanted to do was become the victim of Twilight's wrath, but she knew better than to disobey the Sun Princess. To her utter reluctance and dismay, Starlight said as she bowed,

"Yes, Princess. It shall be done."

"And Starlight?" Celestia asked.

"Yes, Your Highness?"

"When the two of us are alone, you have no need to bow."

Feeling embarrassed at having embarrassed herself, Starlight said,

"My apologies."

"No need to apologize. Just keep following Twilight and Pinkie. I need to know whatever's going on behind my back, so that I can confirm whether or not they are keeping this a confidential mission."

"Yes... Princess." Starlight said, albeit uneasily.

"Just relax and act like nothing has happened between us two." Celestia said as she showed Starlight the door.

Day after day, Starlight followed Twilight and Pinkie to the castle, overhearing whatever conversation they were having. It became a habit that eventually cultivated into obsession as Starlight reported every detail to Celestia. Her suspicions grew more and more as she started receiving despicable news on what Twilight was planning to do with the ooze. Having heard enough, Celestia said,

"I want you to bring me a sample. I need to confirm whether or not Twilight is not planning to use this ooze to boost the power of our armies."

"But Celestia... isn't it a little too extreme to-"

"Starlight... if we don't do this, Equestria will be in even more danger than we know. I don't want this on our consciouses. Please, do this for us." Celestia begged.

Knowing that she meant well and not being disrespected in anyway, Starlight said,

"Okay... I'll blow this whole thing wide open, so that we can conclude whether or not Twilight is doing this for nefarious purposes."

"Thank you, Starlight. I wouldn't know what I would do without you."

Starlight nodded with a smile and left Celestia's room. Like always, she followed Twilight and Pinkie. But this time after they finished conversing with Cadence and Luna, Starlight followed Twilight all the way to a room. She had no idea what was going on behind the door, but she needed to go inside. She moved closer and put her ear closer towards the door. But for some reason, she couldn't hear a sound inside. She slowly opened the door to see what was inside, but when she entered, Twilight was nowhere to be seen. Starlight looked around the room. It was creepy and scary to look at. It was filled with nothing but lab equipment, dead bodies, and the same black, corruptive ooze she's seen time and time again. Starlight looked at the corpses and decided to have a look. Upon closer inspection, she was revolted to discover that Twilight was not only teleporting dead bodies, but she was also using the ooze to experiment on them. Knowing that Celestia needed to know about this, Starlight levitated one of the canisters into one of Twilight's bags. But just when she was about to leave, she bumps into Twilight, scaring the wits out of Starlight.

"Hey, Starlight. Whatcha doing?" Twilight asked in a casual, yet eerie tone.