Gardner was attacked by a small colony of Grindylows.
Grindylows were very common little aquatic demons, probably among the most common kinds. They lived in lakes large and small across Europe, Asia, the Americas, and Africa.
These aquatic magical creatures were cowardly, weak, and yet incredibly troublesome.
Alone, a Grindylow was a complete pushover. It would not even dare catch a fish by itself. If it saw a human or a wizard, it would flee as far away as possible.
But once there were more than three of them, they immediately underwent a transformation.
Just as grasshoppers became locusts when they swarmed, Grindylows also changed temperament when they gathered. They became not only bolder but more aggressive. Some wizards had even run tests.
Grindylows seemed to grow stronger after forming a group, too.
Still, even if research that nobody cared about produced results, nobody paid them any attention.
In short, what people remembered was that a lone Grindylow was useless-the kind of useless that would not dare resist even if you caught it to barbecue it. But once their numbers grew, they immediately became ferocious-the kind that would catch you and turn you into kebabs instead.
Of course, considering that these were aquatic magical creatures, they might only skewer you and skip the grilling.
Harry and the others had studied these Dark Creatures in their third-year Defense Against the Dark Arts classes. There was no particularly good way to deal with them except to quickly take out some of them-if there was a fight, they were bound to be in a group-and frighten the rest into fleeing.
After all, they were still cowardly magical creatures at heart.
Before the event, Harry and the others had specifically consulted Hagrid about the Grindylows in the Black Lake. They learned that the lake's