Ten Demons' Devil Heart is the foundational cultivation technique of the Fire Chieftain. It is a cultivation method rather than a simple "evil technique." The fact that the Fire Chieftain managed to secure a prison cell within the Burning Hillock speaks to the excellence of this technique.
Keep in mind, the majority of powerful Tire-7 and Tire-8 Demon Beasts that enter the Burning Hillock end up as piles of bones. Being imprisoned by the Burning Hillock is indeed an achievement to be proud of.
What Mister Sun wanted was just the "illusion technique" part. Influenced by the technique itself, this part is intrinsically exceptionally eerie and evil. Perhaps due to this, the illusionary power of Ten Demons' Devil Heart is particularly potent.
