During Lolth's big push to break free from her prison, she was able to gather a large number of evil gods, arch-demons, and devil lords on the outside to assist her in her escape. Though the attempt failed, and the forces of evil were scattered, this coalition of evil was not entirely dissolved. The evil god of death, Nerull, took command and led them in a war against the good-aligned gods, promising his followers power and dominion. Nerull himself intended to slay as many gods as possible in order to claim their souls for himself and increase his own power. Many of the good-aligned gods banded together to defend against them, but a significant number of deities across all alignments decided, for one reason or another, not to get involved.
One of the major turning points in the war came when Heironeous met his evil half-brother, Hextor, in honorable combat. Their battle raged on for days, until at last Heironeous emerged, beaten, battered, and wounded, but victorious. And it was in this weakened state that he was assassinated. Though it was Nerull who did the deed, he was successfully able to redirect the blame towards those gods who had previously remained neutral in the conflict. After one was slain in revenge, a number of the others were persuaded to join Nerull's side. Others joined the side of good, determined to prove their innocence. Even after the true culprit was revealed, many of those who had sided with Nerull chose to stay.
In the end, it was Boccob and Wee Jas who found the key to ending the war. Wee Jas confronted Nerull and was slain, and her soul taken. Her sacrifice, however, was all part of the plan; Boccob channeled a powerful spell they had devised through her soul, resulting in a catastrophic explosion that destroyed both Nerull and Boccob. With Nerull gone, the forces of evil quickly fell apart and were defeated.
The war had many consequences, on both the mortal and divine levels. Many gods on both sides were killed, and not all had successors. The planar barriers themselves were affected, resulting in the collapse of the four elemental planes into the Elemental Chaos, as well as triggering the creation of the Shadowfell and the realigning of the Feywild. A number of small scale religious wars also broke out as a result of so many churches, temples, and cults becoming defunct with the deaths of their respective deities.