In Pellatarrum, funerary rites are taken very seriously. Given that 6 hours of every day are spent bathed in negative energy -- which is pretty much the standard definition of "unholy magics" -- making sure that your dead are properly laid to rest and are unable to rise from their graves and terrorize the living is a high priority for survival. Sadly, even when the proper funerary rites are performed, a small but significant percentage of the dead manage to reanimate in one form or another.*
What all of this means is that when a Bonefield appears, it's the result of at least three mistakes in basic funerary protocol, and that level of sloppiness is what gets towns wiped from the map.
After a large battle (and by large I mean a fight between at least two units of at least company-size or greater), sometimes the bodies are not recovered for burial at home. Sometimes this is due to high-order magic rendering the corpses unidentifiable, or because everyone in the battle died and no one was left to take their dead soldiers home. This is mistake number one.
Mistake number two happens when the bodies for both sides are dumped into a mass grave. This happens more times than you would think: if large groups of men have been fireballed until they resemble crispy sticks, it's really hard to tell whose side a person is on, or if you're burying him next to the guy who had fatally stabbed him in the guts. This is doubly true if the people doing the burial are farmers who weren't involved in the battle and would just like to have their arable land back, thank you very much.
The final mistake is what makes all of this so very tragic, because it is so very basic: there is no priest nearby to perform funeral rites and consecrate the land. Even if mistakes one and two were made, this simple act will prevent the formation of a Bonefield. Sadly, the devastation inherent in points one and two pretty much guarantee that if there were any clerics at the battle, they are dead, routed, or accompanying a high-value person (such as nobility) far behind the lines of battle for extended healing in a place of safety.
So what you have now is a large concentration of well-armed and usually highly-skilled men who died in conditions of rage, terror, and extreme agony, who are dumped into a charnel pit with their mortal enemies, and whose sacrifices aren't remembered because there are no tombstones for them and no way for their families to properly mourn them. This is the seed of a Bonefield.
Like all seeds, however, it takes time to grow. For as long as the soldiers have families to remember them, they remain dormant, if troubled, spirits. There is nothing overtly malevolent about the Bonefield; just feelings of general unease about the place. Perhaps the crops are stunted, or the water tastes bitter, or everyone who passes through it during twilight hours has horrible dreams that night. But it is nothing more than a slightly spooky place of rumor and superstition... until the last living person to remember a soldier who died there breathes her last.
Then it is harvest time, and it is the living who shall be reaped.
The dead within the Bonefield, driven by hatred at their mistreatment and whose last thoughts were of warfare and bloodshed, claw their way out of the ground and assemble in military formation. They are not simple skeletons; they are Skeletal Champions, and they will wage war against the living.
Whenever their relentless march takes them to a village, town, or any other population center without suitable defenses. Undead engineers create siege engines; undead wizards and clerics cast spells; undead rogues sneak over the walls to murder the guards and open the gates; and undead warriors swarm in to kill every living thing in sight. But that isn't the horrible part.
No, the horrible part is what happens afterwards: after the slaughter, the bonefielders take the bodies back to the original charnel pit, and bury them in the same unmarked mass grave. This becomes their supply cache for new bodies, and if a bonefielder is destroyed in battle while there is still a body in the grave, his spirit will return to its resting place and animate another body at midnight.The only way to truly destroy a bonefielder is to remove all of the bodies within the pit, and then consecrate the ground. This is much harder than it sounds, because after the initial assault, the town itself is dismantled and used to fortify the burial pit against attack. In addition,while the properties of a bonefield are such that it prevents the the bodies within it from rising as anything other than skeletal champions, there is nothing preventing the creation of incorporeal undead (ghosts, spectres, etc) as a result of the wholesale slaughter of towns and villages. Indeed, many of these undead become permanent guardians of the bonefield, safeguarding it from attack while the rest of the army is on campaign, bringing back fresh bodies for supplies.
The moral of the bonefield is simple: pay respect to fallen soldiers, or they will wreak vengeance upon those who do not recognize their sacrifice by destroying what they once protected.
* While many towns and villages in Pellatarrum aren't happy to see adventurers come tromping through their homes, disrupting the local economy and filling the heads of youngsters with foolhardy notions, they will always roll out the red carpet when they need an undead problem re-deaded. More than a few unethical groups have turned this into a pretty decent racket, with a Cult of the Dark cleric animating the local dead so that the rest of his party can come into town and "solve their problem" for a hefty fee. Needless to say, Paladins of the Light hate these folks with a screaming purple passion.