The event is quite rare, but it may sometimes be mandatory: that is, if it appears when the Nephalem is on the hunt for the Siege Runes. Due to a bug, to get a multiplayer achievement for it, one must be in Campaign Mode (not the Adventure Mode).
When the zone is approached, the player can see a Timeless Prison, which trapped the whole fight aeons ago, both angelic and demonic forces. Deactivating the stasis field will cause the battle to continue as if nothing has happened. The angels and demons will clash again and again in the middle of the zone, while their leaders will remain in the opposite ends of the crescent-shaped battlefield.
Both angels and demons are hostile towards the player, even though they are much distracted by each other. The angelic regiment is led by Erelus, while Volux the Forgotten (Malevolus in beta) leads the demonic horde. While each warlord is alive, his army will constantly respawn: Angelic Warriors and Warscarred Marauders will receive reinforcements without end. The goal is to slay them both, in either order. Killing either warlord will cause the flow of their minions to stop, making fighting the other one harder (as his minions will no longer be distracted by the opposing army). Due to unlimited spawn of enemies, this is a great event to farm the challenges or achievements that require numerous kills.
Interestingly, warlords will both see the Nephalem as a creature of the opposite race, and be sure that the other warlord has sent them:
"A wingless angel? This is what they send to face me now? Die, insect!"— Volux the Forgotten / Malevolus(src)