It was like a giant flare, or a firework, or a giant light, there was no better way to describe it. It wasn't an explosion in the way normal explosions were. There wasn't any pressure, the edge of it was far too defined, the difference between what was in it and what was out of it too great. It was almost like a water balloon, without the balloon, and with light and heat and raw magical energy instead of the water, and far far larger than a balloon, and it didn't pop, it just retreated and dissipated. So it was nothing like a water balloon, but Twilight still used a water balloon to represent it when she had to build a model to explain what had happened. She used a white one with a light inside of it, which at that scale was a pretty good representation of what the ponies in the nearby towns saw.
What they saw was akin to a large, tall dome of pure white light, brighter than the Sun if directly looked at, rising and growing in the middle of the forest over the course of roughly fifteen seconds before fading away. What they saw after that was the crater left where the dome of light had been. Everything in the area was gone, leaving only a perfectly smooth edge of almost glassy soil that curved downwards in a mirror of the upper half. The air there smelt weird, like copper, and at the edges of the crater rocks and trees and branches had been severed as smoothly as the ground had.
Twilight was called there as soon as she heard the news, meaning she invited herself there, but the news was in fairness brought quickly to her, as soon as someone realised the sheer amount of magic involved in the act and got word out to her. What she realised upon analysing the scene was what the ponies there hadn't seen, and she shivered knowing what had happened there. They had not seen what had been at the core of the magic flare, laid there in advance and afterwards quickly removed. They had not seen the pony or ponies who'd laid it there, and almost certainly brought it away, and who'd hidden in the forest and used magic to trigger the event.
It was an impressive display of control and knowledge if nothing else. The spell used had been precisely calibrated, measured exactly to destroy as much as it had. A little more would have wiped out the closest town, and whoever had done that didn't want that to happen. Yet.
It was a message. Clear as day, it was a warning. Hopefully it would be followed by requests, or letters, or anything else. Hopefully it wouldn't just lead to senseless carnage. She'd intensify security anywhere she could, but there was not telling what place would be targeted next. Even if she didn't look too kindly on making deals with terrorists, it was the better alternative compared to having the next scale be used to wipe out a city.