The extinction of the dinosaurs is recent memory. With the exception of the last few dragons, all reptiles were wiped out in a single generation by a virulent fungal disease that killed their eggs. Birds were not affected because their hard shells block the fungal spores.
The ecological niches formerly filled by the dinosaurs and other reptiles are slowly being filled by mammals and birds, In the meantime the vegetation that they would have grazed is growing unchecked turning fertile plains into jungle and driving the human inhabitants, who had previously hunted the dinosaurs there, to abandon their hunter-gatherer existence and take up agriculture. These are the barbarians.
The last remaining dragons have a last few precious eggs that they are protecting from infection in hidden, isolated places that they guard fiercely. They hope that with the extinction of all other reptiles the disease will eventually die out in the wild and they will be able to hatch their eggs and breed again safely.
The part of the world where the campaign begins is in imminent danger of being overrun by hordes of starving goblins who can no longer hunt dinosaurs but are incapable of switching to agriculture for large-scale food production.
The human population is struggling to defend against the goblins at the same time as having to develop an economy based on meat farming. The farmers and the military have competing demands for manpower and resources and governments are not lasting long. At the periphery, local warlords hold power and farmers are subservient whereas In the more civilised areas, where the risk of imminent invasion is less, the leaders of farming collectives hold the reins of power.
The irony is, that the goblins do not want to slaughter the farmers. They want the farming economy to be strong enough to produce enough food for them too. They are trying to defeat the human armies, not so much because they are constantly attacking the goblins but because they are a drain on the farmers' production. The warlords can only retain power while the goblin threat is real, so they arrange for the war to continue. If they packed up and went home, it would solve the problem.
Gods exist and are worshipped individually and collectively as a pantheon. They co-operate and compete with each other for reasons of their own and are liable to intervene in mortal affairs if it suits them. At the time when the campaign is set, some gods are working to promote a peaceful agricultural society, whilst others are stirring up war between humans and goblins. To them, it doesn't matter who wins but how people conduct themselves in the struggle,
At the end of the campaign, the gods will sit in judgement. If the humans have conducted themselves worthily, the goblins will be swept away to another plane leaving the humans masters of their own world. If, on the other hand, human society is found wanting, their armies will themselves be annihilated and the remaining human population will become slaves of the goblins and, in some cases, meat for their table.
