The Plain of Corpses was vast. Before being taken over by the demons, it had been the largest border region of the Kingdom of the Sacred Dragon. After the demons' invasion, the Kingdom had launched multiple battles and only regained a tiny portion of the territory.
The Kingdom of the Sacred Dragon had suffered heavy losses in all those battles. Regardless of how many soldiers they sent, they could not make even the slightest dent in the demons' hold. During the wars, the demons came and went at will, and were completely impossible to track. The Kingdom had put in huge amounts of resources all those years just to locate the demons' nests, but to no avail.
Zhan Fei had initially assigned the Blaze Army to search for the demons' lair purely as an excuse to sow discord between them and the Wolf Smoke Regiment. In reality, he was well aware that being able to locate the demons' nests within such a short span of time was impossible.