The life-bound mosasaur was Xu Qing's first divine repository's Heavenly Dao. It had been formed for a long time. This caused the first divine repository to be filled with spirituality. At the same time, the power of laws and rules in it was also the most abundant.
However, the second divine repository formed from the Poison Restriction, the third divine repository formed from the power of the Purple Moon, and the Emperor Sword Repository were only repositories without spirits.
Spirit was the Heavenly Dao, and a repository without the Heavenly Dao was not complete. The five repositories wouldn't be able to erupt with all the energy, nor could they form rules and laws, failing to become Nihility.
Hence, obtaining Heavenly Dao was Xu Qing's goal in coming to the Divine Realm other than needing the identity of the Grand Mystic Heaven!