What would become the Army of the Lost started as a petty argument between Queen Yocasta and the King of Karnivhal, when a Karnivhalian Countess eloped with a Duke of Vallermoore. The King demanded through his ambassador her extradition so he could have her forcibly married off. The Queen refused point blank, one thing led to another and a war broke out over it, during which several thousand Vallermoorian prisoners of war were tortured and slaughtered.
When the mass grave was found the Queen was shocked and it was then that Sir Edmund Verney , one of her closest friends and a highly skilled wizard, told her that he had perfected a form of the wish spell and he could use it to raise the dead as living people. Though hesitant, the Queen was convinced that he knew what he was doing, and so with the best of intentions the spell was cast. Either because the spell had not been perfected, or because of the latent evil in the place because of what happened there, it went horribly wrong.
Instead of returning to life, the bodies picked themselves up, grey-green with decay, their veins outlined in tracings of blue, still wearing the tattered uniforms they had in life, and marched in the general direction of the unconquered territory. When they reached the nearest Karnivalien army, they charged, and massed fire from crossbows and musketry did very little to stop them, as even if blown apart, the body parts kept on squirming on towards the enemy.
And those who were killed by the zombies, by whatever means, choking or neck breaking or being cut down with their own weapons, rose from the dead and joined their killers as part of the same undead army. When they came across civilians, they killed them too, unless they were children or babies which were left unharmed . In a reverse of normal armies, the more battles they fought the larger the army of the undead got, as it ravaged Karnivhal. It did not loot, it did not rape, it's members didn't get drunk and cause merry hell in the taverns, they just killed.
Their stratagy was not great, they just marched straight at the forces facing them, but as nothing barring total destruction could stop them, enough would allways reach the enemy lines to win the battle. The terrified survivors have fled to a few fortified towns, and the Army of the Lost still marches on. Noone knows what will happen when it reaches the borders of the next country. Some think it will finally find eternal rest, others that it will march on until it reaches the sea.