#normal earth. then one day bam here's the crawling beasts. and THEN monsters happened

[ranticore]

so what exactly is a 'crawling beast of the earth' and why did harpies have to develop such extreme modifications just to defend against them?

nobody really knows what the crawling beasts are except the wyrms, and they're not telling anyone. the beasts take many forms but commonly they are eyeless and worm-like with large jaws but no mouths. they do not eat, they show no signs of intelligence, and it seems that all they do is crawl and bite. their bodies have a high heavy metal content and by their very presence they leach toxic waste into the earth around their burrows that destroys organic life. although nobody really knows what they are or where they came from, it's universally acknowledged that crawling beasts are not monsters or natural animals. monsters - not animals - are able to physically touch a crawling beast without being poisoned and you can kill one by ripping it to shreds, but the easiest way to kill one is to throw their bodies on a pyre.

in terms of behaviour they seem to show no response to injury or environmental stimulus but always crawl or burrow towards the highest concentration of large living creatures they can sense (through means unknown). if one gets within biting distance they will bite.. even if there's a layer of topsoil between you and them. they do not notice others of their own kind and can mindlessly form huge aggregations underneath villages or settlements, fouling one another until eventually the sheer volume and mass of the pile-up spills onto the surface.

the purpose of the beasties, in this setting, is to provide a common enemy that can be a catalyst for human and monster cooperation. they're kind of a macguffin that makes everything else happen, but i like them because they're undefined and terrifying for everybody. one big crawler will destroy a field of crops in one night and make the soil barren, and that can be the difference between making it through the winter and starving, in a small village. crawlers are the reason flighted harpies fear touching the ground. the whole land is infested with them (the sea, too) and their numbers are only growing.

for most harpies, attaining the size and strength (and talons) necessary to fight a crawling beast would reduce their ability to neatly and accurately forage or hunt their normal animal prey. so instead of everyone developing this weaponry, instead only one guy in a flock does, and thereafter he's the flock's bodyguard. kings suck at hunting and foraging. because even regular eagle harpies are naturally pretty big and pointy, they are the first choice for human falconers who want a partner who can swoop down, snatch up a crawling beast, and drop it on a fire. solitary monsters fare the best here since they usually don't attract any crawling beasties, except when they're pressganged by humans (or other monsters) into helping the general pest control effort.

only wyrms are known to hunt and eat crawling beasts.