Douglas pulled Sirius straight off the speedboat, then said:
"Sirius, stop messing around. He's the best choice among us. He needs to draw all the Inferi over so we can deal with them together. Only then can we search for Regulus's body without worrying, and his magic is the weakest here..."
After hesitating for a moment, Sirius directly ordered Kreacher to drive the speedboat. However unwilling Kreacher was, under Sirius's orders and driven by his wish to find Regulus's remains, he reluctantly sat down in the speedboat.
As soon as Kreacher sat down, a smile appeared at the corner of Douglas's mouth. He shouted, "Kreacher, hold on to the handrail. Activate cruise mode!"
The moment Douglas finished speaking, the speedboat started on its own and raced around the lake with a roar...
Wherever the speedboat passed, large numbers of Inferi emerged from the lake and lunged toward it, but the speedboat seemed to have eyes, weaving about with all sorts of flashy maneuvers. Kreacher had absolutely no control over the speedboat. Heart-rending wails and curses echoed through the cavern, especially the word "Mudblood"...
Seeing the others' astonished expressions, Douglas explained:
"This is a voice-control system developed using Muggle technology. That's why you can't sense any magic. Just enter the correct command, and the whole boat will follow a preset route... All right, enough talk. In about three minutes, the speedboat will gather all the Inferi in front of us.
"What we need to do is destroy some of the Inferi and trap the rest. Dobby, find a spot underwater without any Inferi and set up the Magic Array I taught you before ahead of time.
"The three of us will destroy the Inferi leaping above the surface. Those are stronger, so there's no need to keep them around."
They watched