This was a major conflict between the Demons, led by the Death Knight Fost, and an allied force of humans and elves.
Fost experienced victory initially as the Elves and Humans were just as likely to attack each other as they would the Demons.
However, after the Elves and Empire of Man united, they were able to defeat Fost.
The allies separated each piece of Fost's armor, and sealed them away. It was hoped this would keep the Death Knight imprisoned forevermore.
Fost was much greater in power during the Old War, a singular threat able to act on his own.