The reason why the Temple Alliance dared agree to let those demons in was that they didn’t fear letting them obtain more spiritual stoves. When fusing more than three spiritual stoves, the danger would be enormously high and the chances of success infinitely low. And moreover, the Illusory Paradise in itself rejected the darkness element.
However, the Temple Alliance and the higher-ups of the Priest Temple didn’t expect that the Demon God Emperor could meddle in this way, saving those demon powerhouses and ruining their plans totally. And this time, the demon powerhouses killed the great majority of the Demon Hunters that entered. In terms of damage, both parties lost a lot, but the demons lost a lot more. It was after all only successors from various demon gods that died in this trip.