Nerull

Nerull is the 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.

Following Lolth's failed escape, he gathered together a number of evil gods in a bid for power and control. He hoped to cause the deaths of as many gods as possible, both to weaken the celestial power structure and to claim their souls for himself where possible. He personally slew Heironeous, and successfully diverted the blame away from himself, furthering the war. Eventually he slew the other goddess of death, Wee Jas, and stole her soul. It was this act that would be his undoing, however, as Wee Jas had planned for this to happen, and Boccob channeled a spell through her spirit that destroyed Nerull and ended the war.

Realm
Nerull dwells in Carceri, either in its outermost layer or its innermost. According to On Hallowed Ground, his realm is called the Crypt and is a city inhabited by the dead and undead. There, Nerull consorts with fiends of all kinds, who wander the realm devouring the shrieking souls trapped under Nerull's power. According to the 3rd edition Manual of the Planes, his realm is Necromanteion, described as a citadel carved from black ice, where the souls of the dead are trapped within the walls, ceilings, and floors. Demonic clerics perform twisted experiments and recite ghastly litanies. Nerull's throne is within a wide hall called the Hidden Temple, and even more unspeakable horrors are said to be buried in tunnels beneath.

Unlike most inhabitants of the Red Prison, Nerull wasn't banished to Carceri; he lives there because he likes it.

Dogma
Nerull's faithful believe they will be rewarded for acts of murder, for every living thing is an abomination in the eyes of the Reaper.