Will you please explain about Yama Dharma? why he is called Yama Dharma Raja? is he a king?

Yama is the son Surya and younger brother of Shani. he is given the post of administering Dharma or righteousness in the three worlds. Since he sits in the seat of judgement for deciding the fate of Individual Jiva he is called Dharma Raja. He rules over the kingdom of hell where punished souls suffer their penalties.He is like the Jail Superintendent.