Celestial Water Spark
The Darkmoon Tribe can do without the Dark Crystals, but it cannot do with the Water Vein. On the Infertile Plains that run amok with Demonic Beasts, water was equivalent to life. Without water, life cannot be sustained. The humans would all die of thirst.
As such, if the Great Ironhawk Tribe had succeeded in their plans, the Darkmoon Tribe would have slowly ceased to exist if they could not find another Water Vein in the underground to regain access to drinkable water.
Leon began to understand why they would go to such lengths to keep the secret sealed. The Great Ironhawk Tribe wanted to destroy the Darkmoon Tribe, but they do not want to fight an all-out war.
This should also be why the spies no longer tried to recruit him and made attempts on his life. It was not a closed secret that he can cure Demon Possession. If the spies stayed within the tribe for a period of time, they would come to learn this truth.