King of the underworld in Chinese folk religion, taken from the Buddhist Yama: he judges the dead and keeps the book of life and death (生死簿). A cruel, frightening boss is called a 活阎王.
阎罗 shortens the Sanskrit Yamarāja, whose rāja part already means 'king', so 阎罗王 says 'king' twice. To 见阎王, 'go and see Yama', is a joking way to say 'to die'.
usage
The saying 阎王好见,小鬼难缠 means 'the big boss is easy to deal with, but his underlings are troublesome.'