The goblins of this post-apocalyptic world are short, spindly, hairless, gray-skinned humanoids, with oversized heads, mere slits for noses and ears, mouths full of sharp pointy teeth, and massive black eyes. In their natural state, they communicate in hisses and yelps, and are barely above the level of animals. They wear rancid furs and random scraps of metal as armor, and wield whatever weapons they have been able to steal from their victims.
You may have noticed some similarity between the goblins and the demi-humans, particularly the elves. Gray skin and black eyes are the major thread linking them, and the elves have even more goblinoid characteristics. It's not a coincidence.
The goblins are but the servitors of the awful Hive Minds. They are bestial on their own, but when a Hive Mind is present, it fills the goblins with its will, its desire, its hate for the natives of the terrible world it cannot escape.
The Hive Minds are gigantic, tentacled brains, residing in massive jars of briny, yellowish fluid. They rely on their goblin slaves to interact with the outside world, always keeping their physical bodies well away from harm. Hive Mind powers increase as they age. A juvenile Hive Mind is only able to possess one or two goblins directly, and at short range, while only giving general mental orders to the others. A mature Hive Mind is capable of much more, over greater distances, and its powers are capable of influencing, and even killing, humans.
The Hive Minds are jealous of the human use of magic. They are incapable of using magic themselves, and consider it a dire threat. They captured humans long ago, and put them through extensive breeding and hybridization programs with their goblin servants, to try and create a soldier-slave capable of using this magic for their own ends. Their failures were the dwarves and halflings, but with elves they succeeded. The human will was too strong to reliably control, however, and the Hive Minds eventually terminated their experiment, burning the test subjects alive. A few managed to escape, and established the demi-human races in the apocalyptic wastelands of Earth.
The original Hive Minds on Earth arrived from other worlds in extra-dimensional craft that have been bound to the planet by inexplicable aetheric influences. No new Hive Mind has arrived in many thousands of years, and most of those remaining are the spore-bred descendants of the originals. Their technology is lost to them, as it cannot be replicated without extensive manufacturing facilities and materials not to be found on Earth.
There you have it, goblin history in a nutshell, and the horrible secrets of the demi-humans are revealed. I just love brains-in-jars. It adds some Atomic Age cinema flavoring to the campaign. I'll introduce the Hive Minds at some point after the first level, they're a bit too gonzo to fit in with the horror theme of the first level.