The Night war is a bit of a misnomer. It was a war to stop the unending night. It was fought at all hours of the day.
It was an underground war both literally and metaphorically. The main belligerents were the Illithids, whose goals were to turn out the sun. The Illithids lived in the deep Underdark, but their methods were about using proxies and manipulation. They sowed chaos and distrust across the world.
The Night War finally came to an end when the entire Illithid Species took to Space. The rest of the world used the greatest magics that had ever been gathered, a mobilization of foot soldiers more than had ever been gathered before. The heroes never came back.
"Fire and advance, warriors of the light! Step over the bodies of the fallen! Never stop advancing! There is no retreat! Retreat means death for your family! WE WILL CHOKE THEM WITH OUR DEAD IF THAT'S WHAT IT TAKES!"
-General Golrag Trabrash, Goblin of the Dust Plains leading the Paladins of the Unwavering Banner moments before his death.