One versus one battles to claim the ownership of a territory, that was the condition placed over the conquest of the southwestern coast.
Specifics about the battles and the agreements were sent alongside Chasing Demon's message, Elder Austin was particularly annoyed when he read that the Hive would only obtain the desert without fighting since it was situated at the exact corner of their coast.
Powerful sandworms inhabited the desert, and one rank 5 specimen had even been confirmed, it was virtually useless as an asset.
Another aspect of the agreement was that the territories on the coastlines could only feature battles between rank 4 cultivators, while rank 5 cultivators had to fight for the areas more inland.
Simply speaking, Elder Austin couldn't fight for the ownership of the lava lake, he could only join the battle for the azure plain or similar territories that had yet to be explored.