Underground Holy Spring

The Underground Holy Spring (UHS) is an ancient resource in Bo City that assists in the cultivation of mages and is located beneath the city. However, the power provided by the Underground Holy Spring is running out, and the amount of mages that can train there is limited.
Every year, only the strongest young mage in Bo City is allowed one week in the spring to train. The mage is selected among the senior high school students and from the prestigious clans.
Season 2/Chapter 90 Spoilers 
The UHS was the main reason that the Black Vatican attacked Bo City. Yu Ang was planned to enter the UHS as the strongest young mage and steal the UHS for the Black Vatican. However, all of this changed when Mo Fan defeated Yu Ang and was the one to enter the UHS instead. This forced the Black Vatican to change tactics and proceed in the more forceful annihilation of Bo City.
Lin Yu Xin, the vice captain in charge of protecting the UHS activated the sequence to bottle up the UHS in times of emergency. She then handed the bottled UHS to Mo Fan for him to bring it to the safe zone. She left it in Mo Fan's possession in the assumption that the Black Vatican would target her instead of Mo Fan.
During the escape to the safe zone during the catastrophe of Bo City, the vanguard squad was stopped right before the safe zone by Officer Bai Yang of the military. While Bai Yang offered to take the UHS for protection, Mo Fan was suspicious of him and refused. When confronted, Mo Fan offered the UHS to the loach pendant and claimed to drink it rather than hand it over. When Mo Fan did this, Bai Yang revealed himself to be a member of the Black Vatican and attacked.