Every 777 years the Bad Moon waxes. It's unholy power infuses the idols of the Night Goblins, driving them forth with the Goblins hatred for all sunwalkers. The curse lumbers alongside their armies, spreading their misery and hate among all living beings.
The background of the Bad Moon was recently revealed in Age of Sigmar. Gorkamorka and Sigmar were having an eating contest, and an average moon in the sky of Ghur was bitten by the Greenskin God. Luckily for the Bad Moon, that was when Gorkamorka got full. Shards of the moon fell to Ghur, forming mountains that the Night Goblins infested. The Moon itself was now shaped like a Grot's head from the bite, and was infused with Waaagh! energy and brought to life as a god in its own right.