Ever since being driven off the Board of Governors, Lucius Malfoy had been unwilling to simply accept being "out of the game." It was only to be expected that he would appear at the school again as an official from the Ministry of Magic.
Though Pomona and Filius had both prepared themselves for a "fight," Albus sent them, along with Snape, to the lake by the Forbidden Forest where Harry Potter had cast his Patronus. Not only had it driven away the Dementors, but it had also protected his godfather.
The Centaurs had noticed something strange there when passing by last time. Unlike humans, whose clothing prevented them from keenly sensing changes in temperature, they could tell that the air there was still very cold despite it being midsummer, even though the lake had not frozen over as it had on the night the Dementors appeared.
In short, the three of them had an excuse to leave the castle, leaving Albus and Minerva to confront Lucius Malfoy head-on.
The tangled roots in the forest were very unfriendly to Flitwick. Places the others could step over in a single stride required him to use certain "little tricks." Fortunately, the lake was not far away, and they reached it before long.
Rather than a lake, it was more like a somewhat larger pond. It looked tranquil by daylight, its water shallow and clear enough to reveal the stones on the bottom.
Muggle eyes could not see Dementors, but they left behind faint shadows, like transparent blackness. When Muggles saw those shadows but no "physical form," they tended to make a fuss. Many "hauntings" began that way, though not every "ghost" had anything to do with Dementors. By regulation, Dementors were forbidden from entering Muggle communities. Boggarts, on the other hand, were difficult