The reason for Cao Guang's excitement was a single liquid Water of Kun as it was locally called.
Cao Guang knew that these familiarisation missions would take them to the villages of Ancient capital. Only these villages had a speciality they stood alone pretty much unprotected among a sea of Undead yet nothing happened to them.
If Cao Guang's memory wasn't wrong then the chiefs of these villages were only at the peak of Basic tier. Yet they were safe from the Undead because they drank the Water of Kun.
They were all descendants of the Qin Emperor and were tasked with protecting the Water of Kun and were allowed to drink. The Undead who were also his descendants therefore ignored this group of people.
Although widely considered the greatest mage of the Chinese lands , Qin Shi Huang had long switched teams and had become the founder of the Undead element in China. He was also the Ancient King of Undead and now an Emperor level Undead.
His understanding of Earth and Space element had long been outstanding. He had been the King who commissioned the Wall and they held his secret even to today. Yet time had not been kind to him, when he was aging he used his final element to his advantage and became an Undead and therefore one humanity's greatest enemies.
But his shining light had failed him when he was reduced to a Soul Battle Robe after a battle in the Underworld for Supremacy against and old foe for the position of God of Undead. It would take a Zhan Kong to infuse some much needed blood into his soul and revitalize him.
He would then go on to wrestle with Zhan Kong for his body and would have a compromise with him and bide his time. Zhan Kong had only been a Super mage and the Ancient King would erode his soul over time and win a body for himself again.
Yet there were a couple of questions Cao Guang was interested in about the cultivation of these historical giants, but answer were illusory and not attainable right now. So the Water of Kun was Cao Guang's focus now.
The Water of Kun was also the same as Underground Holy Spring, this made it an important resource for his Little Tora pendant. It could be used to improve to a Star Nebula magic tool and therefore his cultivation level.
For this reason he had acquire Water of Kun from these villages even if he had to steal them. He would leave enough for these villages to survive but he had no soft feelings for them, for he had long figured out the true nature of Water of Kun and it's variations.
The break through for him was the fact that Bo city's Underground Holy Spring had lessened with use and needed them to rationise it's use to once a year and once every few years in the future.
But these villages were able to support the consumption of their entire population for a long period of time and never run out. This was unlike any other known source of Underground Holy Spring.
When he figured a plausible answer for his questions he figured out the true nature of the Water of Kun. The answer lay with the Qin Emperor and his elements namely Earth, Space, Psychic, Chaos and Undead all at the Forbidden Curse level.
He had made the Great Wall of China with his expertise in Earth, Space and Psychic elements. He had built the dark abyss using his elements and turned himself to an Undead Emperor as well.
He had laid down formation beneath Ancient Capital to produce more Water of Kun through the bloody ritual the living and Undead went through every night. When either of those were killed their Soul Essences would be released to the world, A portion of it would become new Undead and another would be absorbed and condensed into a liquid, The Water of Kun.
Cao Guang's conclusion stemmed from many facts and inspecting his Little Tora pendant. It could use Underground Holy Spring and Soul essences as it energy source. Soul essences were also among the purest source of energy.
Mo Fan's Little Loach pendant was able to also absorb the Pharaoh Spring another source that was formed with Soul Essences, when they explored the pyramids Mu Bai concluded that high ranked mummies and Pharaohs enjoyed the slaughter of the living so much because they could use to make them stronger by just sleeping in their coffins and absorbing the energy of Soul Essences.
Qin Shi Huang had done the same thing. He layed an impressive formation underneath the eternal battleground he had designed and using Psychic spells suggested to the appointed guardians to not to move away from the Waters they were protecting.
Cao Guang also didn't think that the Qin Emperor invented the Underground Holy Spring and had probably reverse engineered after observing a few locations that naturally produced these pure sources of energy.