The Goblin Army event can only happen in the Town of Ravencrest and certain criteria has been met
If you have not completed the Goblin Army event yet and the above criteria has been met when you enter the town of Ravencrest there will be a Goblin Scout surveying the area. If you defeat this goblin scout the Goblin Army event will be triggered.
If you let the Goblin live the goblin will report that the town is guarded by a warrior and not to attack the town.
If you have already defeated the Goblin Army Event then the chance for this Goblin Scout to appear when entering Ravencrest is 1/10
More on the Goblin Army event can be found here. https://terraria.wiki.gg/wiki/Goblin_Army
Any information not explicitly stated on this page uses the default vanilla behavior from the wiki page above.