The biggest goblin invasion ever was approximately 150 years ago, and they nearly overran the entire island. There capital was in the abandoned duergar cities.
The invasion ended for unknown reasons. They certainly had defeated every native army by sheer numbers. But a storm known as the Wrathful Storm blew in from the northwest, and when it had blown off of the Vusclands, not one goblin was left alive. Their numbers have recovered since, but they have never posed as big a problem as they did then.