The heroes did what they could to try to stop the kid from running, but they weren't able to stop him.
Their attacks just didn't have the same effect that Percival's did.
However, Percival also didn't release a second attack as he just watched the kid disappear.
It wasn't that he didn't want to, it was that he wasn't able to do so.
There was a limit to his attacks and he had to recover first before he could release a second attack. Which was why he had no choice but to watch as that kid disappeared into the ground.
"When the time comes, I will take my revenge."
That was the last thing that was said as the kid sunk into the ground.
When it was all over, the heroes continued purging all of the black ooze that was left behind.
Even though the kid disappeared, the black ooze that he had sunk into remained there. Which was why they did all that they could to make that black ooze disappear.
