From Better Than Wolves Wiki
Soul Flux was added as a fourth ingredient for crafting Soulforged Steel, restricting the creation of said steel until players have successfully defeated The End.
Keep in Mind
- Soul Flux doesn't react to heat by itself, and can be left in a stoked cauldron with no consequences.
- There is only a finite amount of Endstone initially generated on any given map. However, this amount is still very large, and it very unlikely that players will consume all of it during the course of a game. Also, Endermen occasionally spawn in the End holding Endstone, which they will place after a while, making Endstone a renewable resource.