After locating a "Magic Mirror" of sorts to another world, Sanies decided to use a spell that would make the guards unconscious so he can access the mirror. Sure he could have turned them into stone, but that's not as pragmatic and there are only a few creatures and magic-users that know petrification magic, so it would be easy to single him out if he turned them into stone. Sanies thought to himself as he made his way into the mirror. It was a genius plan, go into another world, find an alternate universe version of Celestia, take over her body while the real Celestia was having a deep sleep (mainly as a result of his latest attempt at practicing the spell he's using to take over her mind). When no one was looking, he gave out a little chuckle, excited about how well his plan was going. Though when he entered the mirror, he was about to get a surprise.
"Well, this was unexpected"
When he looked down at his new form, he realized he has paws instead of claws and his scales are now all furry. When he looked into a convienently placed puddle, he was pleased to see he still has his wonderful smile, even if his features were a bit more rounder. A weird ape thing then noticed Sanies' new form and decided to pet it. But since Sanies thought the ape creature was going to attack him, he did what he did best.
"Kneel"
The creature, eyes glowing the same shade of green as Sanies' eyes, complied. Sanies then took a moment to observe the creature, noticing the blue skin and the long wavy dark blue hair. While staring at the creature, whose jaw has dropped a bit and is now drooling, he noticed a familiar moon-shaped symbol on her shirt collar. Where has he seen that symbol before? He then took a moment to enter the creature's mind, looking through her memories and realizing that the creature kneeling before him was an alternate version of Luna.
"Hmm, had I known how easy it would be to dominate the minds of the princesses in this universe, I would have done this sooner"
He chuckled at his statement. Princesses? There not even princesses in this universe! Tartarus, there not even close to royalty! Nothing more but a pair of simple teachers. While he knew getting another Celestia would be easy, he never expected it to be that easy. He then commanded the "princess" to get up, while obviously hiding the glowing eyes in order to avoid suspicion.
"Now then, let's go for a little walk"
It was just another average day at Canterlot High (well, as average as it could get when people getting corrupted by magical artifacts or sirens is an unusual occurrence). Really, the only thing that was out of the ordinary was Luna had her head down most of the time and a dark blue Bull Terrier with a green underbelly following her. Celestia decided to check on her sister to see what was up with her.
"Luna, are you oka..."
Before she could finish her sentence, Luna looked up at her. Eyes glowing a sickly green, the green glowing...stuff...from her eyes blasted into Celestia's eyes (it's less awesome than it sounds) which knocked the both of them out. Sanies looked amused, as he did not know he could do that. Learn something knew every day, I suppose. Celestia then got up in a zombie-like manner, her hair disheveled and her eyes...you get the idea (or should I say eye-dea).
After bringing the fake Celestia to Equestria, where she rather conveniently turned into an exact replica of her Equestrian counterpart (glowing eyes and messy hair not withstanding, but that could be remedied) Sanies enacted the next stage of his plan: Pose as the princess to trick Luna and Discord to resurrecting that idiot Storm King so he can use him as a distraction so he can find those who's expertise in mind magic rival that of his own and persuade them into joining his side. While Discord is no longer an option, thanks to Tirek already using and then betraying Discord, and Sanies' magic isn't strong enough to take over the alicorn sisters' minds (just putting them into deep sleeps instead) clearly he needs to keep those two busy while he scours for ponies himself, using the guise as the princess of course. While his original plan was to get inside The Great and Powerful Trixie's head, believing that whatever power she has will be his, he thought up of a backup plan in case Trixie was only "Great and Powerful" at mere illusions and prestidigitation (because if someone is that great and powerful, clearly they don't need to make it into a full-blown title). He will travel back into that other world and use whatever device the purple ape was using to turn into an unusual hybrid between an ape and an alicorn, well, that's what it looked like in that other Celestia's memories anyways.