Guan Yu, a legendary historical figure and a prominent character in the classic Chinese novel "Romance of the Three Kingdoms," became deified and worshipped as a god due to a combination of factors:

1. Virtuous Character: Guan Yu was known for his exceptional virtues and qualities such as loyalty, righteousness, bravery, and a strong sense of honor. He was revered as a paragon of virtue and became an embodiment of these values, earning him admiration and respect from people.
2. Historical Achievements: Guan Yu's military exploits and achievements during the tumultuous Three Kingdoms period in ancient China contributed to his fame and reputation. He displayed exceptional military prowess and strategic skills, serving as a general under Liu Bei, the founder of the Shu Kingdom. His unwavering loyalty to Liu Bei and his significant role in battles elevated his status in history.
3. Folklore and Legends: Over time, folklore and legends surrounding Guan Yu's life and deeds expanded, further enhancing his image. These stories portrayed him as a mighty warrior and protector of the righteous, emphasizing his martial skills, wisdom, and even supernatural abilities.
4. Imperial Patronage: Guan Yu's deification was also supported and promoted by imperial rulers and emperors who sought to associate themselves with his virtues and military prowess. Imperial patronage helped solidify his status as a god and popularized his worship among the masses.

Due to these factors, Guan Yu gradually came to be worshipped as a deity associated with loyalty, righteousness, protection, and martial prowess. Today, he is widely revered and worshipped as a god in various forms of Chinese religion, including Taoism and popular folk beliefs. Guan Yu's deification illustrates the blend of history, mythology, and cultural beliefs that have shaped the worship of deities in Chinese culture.