Goblin Park
by RipvanwormerAccording to The Elves of Alfheim, 300 years ago a horde of 10,000 goblins or goblin-like creatures burst from a magic point into the forest of Alfheim. Supposedly they were refugees fleeing some enemy in an alternate universe. The elves slaughtered them.
I was curious to see if there was something going on in one of the other D&D worlds that might inspire a horde of goblins to flee to Mystara. Well, sure enough, 300 years ago was approximately the time of the First Unhuman Wars in the Spelljammer setting, when the space elves were warring against the space goblinoids, so in theory the goblins could have been fleeing the elven imperial armada only to run into a forest full of Mystaran elves ready to shoot them full of arrows. And if the space elves are still trying to get through 300 years later, won't the shadow elves have a surprise if they enter during their reign?
In the Astromundi Cluster, the illithids were waging the Tentacle Wars, and I suppose some goblins could have been fleeing from that.
There was also a massive orc horde in the Five Shires almost simultaneous to the goblin invasion of Alfheim, and I don't know if there's any way that could be related. Like maybe they were refugees from the Unhuman Wars too?
In the Forgotten Realms about 300 years ago what was happening was the Battle of the Bones, when a horde of 200,000 goblins and orcs were defeated in their attempt to invade the North. Could some of them have escaped to Mystara?
But most promising, I think, was the module Into the Dragon's Lair by Steve Miller and Sean K. Reynolds. This was a very early 3rd edition module - so early that it had to include a conversion guide so that people would be able to play Forgotten Realms priests using the 3e rules, and it referenced features of the 2nd edition Planescape cosmology. It was probably written as a 2nd edition adventure at one point and converted to 3e before publication.
Anyway, the premise of Into the Dragon's Lair is that there's a demiplane with a goblin city called Grodd on it. And an adjoining demiplane with some elves that have been trapped there for centuries. And an imprisoned dragon. I don't think it would take a lot of work to link Goblin Park to Grodd, and say that the incursion 300 years ago was sent by the dragon Nalavara in vengeance for being banished there by Mealiden Starwatcher and his allies at the time of Alfheim's founding. And replace references to Cormyr to Alfheim and Darokin.