These were just a small group of Purgatory members who decided to stay and help the beast. It was obviously a suicide mission, since since the disappearance of the organization's leaders, there was no longer a powerful being who could combat the demigods on the continent.
Their mission was to create an opening for the beast to escape, and they had succeeded. They knew they wouldn't escape from Crhono and his group, so they didn't even try to flee or resist.
They immediately went into sacrifice mode, and although they didn't believe they would succeed, they reasoned that if they suddenly sacrificed themselves, they could take out or injure some of Crhono's allies.
Although weak, they were still demigods, so their self-destruction was not insignificant. The energy explosion they created with this move was similar to a high-powered atomic bomb on Earth. Miles and miles of lush forest were transformed in seconds into a dead, burning desert.
