Enter the Spiritual Pedestal Tier

The reason why the Jiang Clan was willing to take out the key to the Sagittarius Palace Hall was because the Jiang Clan was getting increasingly weaker as generations passed and their power in Extreme Yang City was close to vanishing altogether. They were just barely surviving. Even if they had their clansmen bring the key into the Twelve Heaven Mystery Realms, they'd simply be swallowed whole and robbed blind by other stronger people since they were much, much too weak.

Thus, they decided to take the risk and find a most powerful son-in-law to turn the situation around with his help.

Just think, the Prodigy Tournament had just ended at this time, so this most powerful son-in-law had to be a member of the Prodigy Roll, so he'd have great potential in the first place. Added together with the inheritance inside the Sagittarius Palace Hall, it was practically guaranteed that he would break through and become an ultimate cultivator of the Shattering Void Tier.