The county of Azerath is a a fairly peaceful place composed of many races and is the center of trade for the Bulman empire. Unforcantly the empire has fallen on hard times hordes of orcs, hob goblins, and barbarians from various tribes are sweeping through the Empire destroying all in their path. Emperor Maximus Bulman has pulled all troops from the area to fight a civil war against power grabbing nobles leaving no defense against the hordes but paladins of various deitys have begun recruiting in many towns to fight the hordes. However many have either fallen back to the imperial capital or are too concerned with toppling the local government to care.
A corrupt order called The Knights Of Greed have been taking advantage of the situation by capturing innocents to sell as slaves to sell at the capital. The local trade guilds have used the lack of law to try and destroy each other using the help of mercenaries. Remember there are no prophecies for the players to fulfill only their own ambitions.
Expanding the scenario:
Azerath is a pocket kingdom coming apart at the seems. The old nobility have clung to the notions of absolute power, while the rise of the mercantile guilds, the growth of militant church orders, and a weak army based off of mercenaries rather than conscription or a professional military have undercut everything. If all of the factions involved in the kingdom were to cooperate they could stop the horde and reclaim their lost lands. This is stymied by the fact that each faction claims the right to lead, and that the others should show proper deference. The other matter at hand is that each faction feels that if they capitulate to anyone else, that faction will retain, or will claim supremacy after the horde is defeated.
The greenskin horde approaching Azerath isn't particularly strong, nor is it very effectively led. Rather, as it has advanced, it has faced weak, scattered opposition, or has actually been aided by human factions. When the PCs come into the scenario they are going to be petitioned and otherwise recruited to join at least two of the available factions. The self proclaimed Emperor has a large contingent of soldiers, the bulk of which are mercenaries. The mercenaries are of the less than reputable sort as they work cheap, and many are accepting payment in terms other than coin. The rest of the army is formed of conscripts forced from the prisons and jails and rousted from the poor areas of the capital city. The paladins of the faith are the most competent and potent force, but they are likely to be the smallest. In addition to their magical and physical prowess, they also have a good deal of dogma and ethical requirements that do make them tedious people to work with. The Guilds and the lesser nobles are in fairly equal positions, they have moderate sized groups of soldiers, the guilds fielding yet more mercenaries while the lesser nobles have house guards and a smattering of mercenaries and conscripts.
The Knights of Greed are a problem, as their efforts are actively undercutting the other factions and setting them against each other. The standing plan is that once one or two factions are broken, the Knights of Greed will coerce the remaining factions to work under them and will usurp control of Azerath. They are principally working against the paladins and the lesser nobility. Once one of those factions folds, the Knights will 'pledge' their support to either Maximus Bulman, at a cost, or to the Guildmasters, again at a cost.
The PCs need to play the role of peacemaker and bring at least 3 factions together so that the horde can be repelled, and then deal with the Knights of Greed assassin and saboteurs who will invariably do their best to discredit, defeat, or simply kill the PCs.