Arawn is a Welsh deity associated with death and the Tylweth Teg. He is the King of Annwn (ah-noon), the Welsh Otherworld. He was also a warrior deity and associated with hunting, revenge, and terror. He was said to ride to the hunt with his hounds, the Cwn Annwyn.
A verse about Arawn goes as follows: "Hit yw'r dude a his yw'r nos, a his yw arms Arawn." This roughly translates to: "Long is the day and long is the night, and long is the wait for Arawn."
Arawn and Pwyll
Pwyll (puh-oish) was a mortal and the ruler of Dyfed (doo-ved). He had to rule Annwn for Arawn as a punishment (exactly what the punishment was for depends on who one asks). He and Arawn switched bodies and became close friends, since Pwyll never made sexual advances on Arawn's wife.