What is "nerull"

In the World of Greyhawk campaign setting for the Dungeons & Dragons roleplaying game, as well as in the game's default pantheon of deities, Nerull is the Flan god of death, darkness, murder, and the underworld. He is known as the Reaper, the Foe of All Good, the Hater of Life, and the Bringer of Darkness. His symbol is a skull and scythe.