Twilight stepped back out of the portal, and immediately closed it behind herself. Truth be told, despite the major hiccup she'd run into, things had ended up playing out better than she'd ever expected they would. She was going back to Equestria with much more information than what she could have ever reasonably hoped to find simply by sneaking around, with plans and ways to learn yet more things about the world she was leaving, and with a couple of allies there as well.
Her horn lit up, and her magic aura dug a shallow hole in the sand where the portal had been. She retrieved the magically sealed box she'd left there, and the piece of parchment inside it. Her way of warning Celestia things had gone well, and she could come open the portal to get her out of there. Even if someone from the other side had somehow found the portal, and somehow found that box too afterwards, which in itself was quite unlikely to happen given it was directly below the portal, and then managed to open the box, they wouldn't have known what the password she'd set up with Celestia was. Any attempt at sending a message with the enchanted parchment that didn't begin with that would have immediately told Celestia things hadn't gone as planned.
Twilight was about to start writing, but something interrupted her. A sound. Or maybe a feeling. It was hard to understand properly what it was. Like in a dream not fully realised, an ill-defined sensation she couldn't quite place her hoof on. She turned slightly and looked. "You," she said, but did she know who it was? It seemed more like a vision.
It didn't answer that. She. Twilight was fairly certain it was female, among the many things she wasn't sure about. Instead she looked at Twilight, and passed her something. A sheet of paper, covered in writing. It seemed to become much more concrete than the rest of the vision as Twilight took it into her magic and looked at it. "Read it," the vision said. "Keep it with the other notes if you need to. Understand it. Ignore it. Remember it. Destroy it before anyone else can see it. Use it, when you'll need to, and don't question where it came from. Assume it came from you. Recreate it, if you have a need to."
Twilight looked over the paper again, then around her at the empty desert she stood in alone, as she had since her arrival there. Putting the notes she'd gotten from the unicorns back away, she focused on the enchanted parchment again and, before writing, for a moment thought about how exactly she'd frame things. Celestia wouldn't be happy about her decision to return to that world later on, especially so considering where else she was planning to go and what else she was planning to do.
She'd spend some time with her before going again. She deserved it. Hopefully, it wouldn't be the last time they'd get to spend some time together.