Eh, that's why I raised it. We work out a flavour as we go, what fits and what doesn't. Just need to figure out what exactly the Shadow Curse is, and why it's enough to fight wars over rather than send an adventuring party to kill the monster. It can't just turn people into monsters, or it's an environmental threat; people bond together to fight monsters, rather than splitting apart.
After all, nobody disagrees about stopping trolls rampaging down from the mountains, they just might disagree on how.
DH might have it though. What if the Shadow Curse isn't the motive, it's the means? What if it's both? A war of assassins and clandestine action...
Once upon a time there was just one kingdom, Selridge. Not too long after the Iron Prince established a foothold in these lands, driving away the Elves, other human factions moved into the surrounding countryside and began to carve out their livings. One of these, larger and more glorious than the Principality, was Selridge and its line of just and wise kings. Like any other kingdom, it had its wars, its peaces, its internal squabbling. Life went on.
Around two hundred years ago, the balance of power shifted with the birth of a sorcerer into the ranks of Selridge's nobility, an extremely rare event. Born into a Great House (though not the Royal House) of Selridge, the sorcerer was set to become a sorcerer-Duke. As befit his station, he received the best training in war and politics, in grace and charm and in classical education. The tale tells that he was handsome, to boot, and charismatic beyond measure.
Nobody knows if it was his magic that commanded such loyalty from his followers, or simply his presence. He made friends and alliances at court and everything pointed to an engagement to the Princess, heir to the throne and extremely eligible. Yet when the night of the ball came for her debut, the Princess could not be found. Three days later her body washed up on the banks of the river. The court was shocked, first by the apparent suicide and second by the realisation that this left the succession wide open; no immediate heirs of the Royal Line were available. The sorcerer was assumed inconsolable, for he would speak to none and retreated to the solitude of his own demesne.
Two weeks later, the Duke his father was found with his throat cut, and rumour of foul play abounded. The new Sorcerer Duke accused the Great Houses of the act, of an ambitious soul desiring to take the throne and weaken the other Great Houses by this treachery. Other Great House members were soon struck down by assassins, and it was not long before (rightly or wrongly) the Sorcerer Duke himself was targeted and nearly killed in a fire. The blaze scarred him horribly, and he took to receiving visitors in shadowed quarters.
Injury, paranoia and perhaps grief drove the Sorcerer Duke to madness. He created his own army of spies and assassins, the Curse of Shadows, forging dark tools, poisoned weapons and cloaks of shade and silence to aid them. It was not long before the remaining Great Houses were united against him, and he seceded from Selridge to form the Duchy of Falken. All seemed set for war, when the Sorcerer Duke simply disappeared. His cousin, of the House of Falken, took over the Duchy and was able to prevent open conflict, but the many wounds on both sides made him unwilling to bring the Duchy back into the fold. The Shadow Curse, for their part, broke apart. A core stayed with the new Duke of Falken, while others went independent and many defected to Falken's very rivals in Selridge.
For many reasons, Selridge and Falken exist in a state of permanent cold war. There is almost endless conflict both between and within them, but the legacy of the Curse of Shadows, the assassin's guild (for want of a better word) that loosely monopolises intrigue within the two states ensures that most of it remains on a small scale (albeit an extremely violent one). Atrocities are committed, then instantly covered up. Men are killed and swiftly forgotten. In a land where anyone might be part of the Shadow Curse, might be listening or ready to kill you for the smallest fee, there can only be constant fear.
(In effect, there ends up being a surplus of assassins in Selridge and Falken, to the point where even local figures of wealth and such could hire one for business ends. An extremely paranoia-inducing situation.)