The Eternal Battle is a long-lasting war in the Fissure of Woe between the Eternal Army of the god Balthazar and the Shadow Army of Balthazar's half-brother Menzies, the Lord of Destruction.
The origins of the Eternal Battle remain unknown but it has been fought for an indeterminably long time between the Eternals and the Shadow Army. It was only around 1072 AE that the Shadow Army began gaining an upper hand with turncoat Eternals who had sided with Menzies. The loyalist Eternals eventually pushed the Shadow Army back, however, and brought down the treacherous Eternals with help from mortal heroes, turning the conflict in Balthazar's favor for a time.
Menzies himself remained elusive, though, and continued the conflict while also supplying forces to his later allies Abaddon and Dhuum around the time of Nightfall only to lose these reinforcements as well. Balthazar himself had more or less been idle since the Exodus of the Gods, suggesting that he may not have partaken in many campaigns against his half-brother's army at the time.
Balthazar recruited the Eternals to serve him as his Forged army in the campaign against the Elder Dragon Kralkatorrik in the Crystal Desert in 1330 AE. Based on the Herald of Balthazar's writings, the status of the Eternal Battle appeared to have shifted to allow the Eternals to focus their efforts on the Elder Dragon instead of Menzies's forces.
When Kralkatorrik fell into the Unending Ocean from a mist portal opened by Aurene, he brought several chunks of land from the Mists with him that buried him and formed an island known as Dragonfall. The Burning Forest of the Fissure of Woe was among the islands that fell from the Mists with Kralkatorrik, and several forces from Menzies's Shadow Army were present there. The members of the Order of the Crystal Bloom working against Kralkatorrik on that island mentioned that the Shadow Army were cursed to forever hate and fight Balthazar and his Eternals. With Balthazar's magic consumed by Kralkatorrik and Aurene, the Shadow Army's attention and hatred had turned to the two dragons, the Branded and the Order of the Crystal Bloom.
The fate of Menzies himself remains unclear.