Er Lang Shen (二郎神), a Celestial Official Title (道教神名) for Deities who is strongly connected with the element of water & slaying of demons or Negative Entities.
In the whole Chinese Deity System, there are a total of 3 Er Lang Shen.
Each of them is recorded in different era.
Here, I will do some brief introduction on the individual one:
1 – Li Er Lang (李二郎), son of Li Bing (李冰), a Shu General (蜀將) during the Warring Period 戰國時代 (around BC250).
During that era, due to lack of facilities in opening of new farms, directing of new canals, etc. Li Bing and his son Li Er Lang volunteered to assist the commoners in performing the tasks. Soon the tasks were completed.
And in one year in that era, snake demons appeared in the canal where they had opened, in-order to get rid of the disasters, Li Bing and his son Li Er Lang went to the canal side to fight with the snake demons and soon, the demons were subdued.
From then, the commoners respect them as Deities and after they passed-away, the commoners/folks dedicated Temples to them.
In later era, Li Bing was given an Official title of Guang Ji Wang (廣濟王) and his son Li Er Lang with the title of Hui Ling Hou (惠靈侯).
2 – Priest of Mt Qing Cheng Zhao Yu (青城道士趙昱) aka Zhao Zhong Ming (趙仲明), born before Sui Dynasty 隋朝 (AD581 - 618), was once an Area Magistrate Officer.
After seeking Master-ship under Master Li Yu (李), Zhao Yu went to Mt Qing Cheng to do his self-cultivation.
One year, in Jia State, a gigantic water demon appeared in one of the country side river, upon knowing the existing of such a demon, Zhao Yu came out from Mt Qing Cheng to Jia State to do the slaying of the demon.
After several rounds of battling with the demon, Zhao Yu managed to subdue the demon.
From then, the folks dedicated a temple to him near to the mouth of the river, which later the folks addressed him as Guan Kou Er Lang Shen Zhao Yu (灌口二郎神趙昱)
Now in China Si Chuan Province (四川省), you are still able to find the temple dedicated to Guan Kou Er Lang Shen.
3 – Er Lang Shen Yang Jian (二郎神楊戩).
This is a newly known Deity after the classic Feng Shen Bang (封神榜) appeared during the Late Ming Dynasty (晚明年間).
In Feng Shen Bang, he was known as the Leading person of Mt Mei (梅山首領), which later subdue the 6 demons to be his assistants – later people addressed them as Mei Shan Qi Sheng (梅山七聖).
In some myths, Yang Jian was known as the nephew of Jade Emperor (玉皇大帝之姪).