After spending a few days, Du You refined the core of the Flame Demon, eventually leading to its death. He knew the Flame Demon wouldn't die completely clean because as long as the foundation existed, it could revive.
To fight so far from Hell, the Flame Demon certainly wouldn't leave nothing behind.
In fact, even Du You left his own mark on Earth. This was to prevent death so he could be resurrected. Of course, Du You only left a mark and not a fragment of his soul.
After all, he can still improve now. If he leaves behind a fragment of his soul, it would be troublesome to continue improving.
By only leaving a mark, revival is possible but if something really goes wrong, the time to resurrect would be very long. Similarly, being able to resurrect by leaving only a mark is a privilege Du You received due to his significant contributions to Earth.
Even those who reach tier nine might not enjoy such a privilege.
