Without a doubt, the lineage of the Molten Furnace Wizards is powerful, especially after acquiring the "Sacrifice Method." They can gain knowledge and power without paying the horrifying price demanded of Evil God believers, which explains their rapid expansion and their penchant for courting death.
However, this lineage also has clear flaws.
Serious cultivation?
That is utterly useless. The only ways to enhance their strength are to add fuel or to sacrifice to the Evil God.
In the Dark Age, the Mysterious Side was at least ten times more terrifying than it is now.
According to those ancient records, it was an era where monsters were everywhere, and ordinary humans lived in constant peril. Even powerful beings like the Immortal Earl and the Twelve Saints couldn't traverse the lands shrouded in darkness.
Perhaps it was this environment that forced the Molten Furnace Wizards onto a radical path.
