Tian Wang

Tian Wang (天王), translatable as either "heavenly prince," "heavenly king" or "wearer of the blue apron" was a Chinese regal title that was most frequently used during the Sixteen Kingdoms era, among the kingdoms founded by members of the Wu Hu tribes, often used as an intermediate stage from claiming a prince/king (王, wang) title to an emperor (皇帝, huangdi) title. However, for most intents and purposes, a tian wang was treated as an emperor—their spouses carried empress titles, their styles were the same as emperors, and their posthumous names and temple names reflected imperial status. The only difference appeared that when the head of the state carried the tian wang title, the highest rank among nobility usually became duke (公, gong) rather than prince.

...
Wikipedia