Elder Beast

"When you're a kid and someone first tells you about the Elder Beasts, you don't have the frame of mind to understand them, so you think of something like a dog the size of your house, or if you've met one before, a dragon. I think even the people in the cities can only imagine them as, say, big blobs of flesh with lots of eyes and tentacles. Not a tree endlessly sprouting more trees from its bark until its body becomes a forest, constantly warping into a spiral that tries to lure you in. Or white tendrils with black borders, like you're being chased by your little sister's drawing, except that this is the truest form of the color that you can't find in nature, and then it starts seeping into the ground...Because that's a lot harder to imagine, and it's even harder to imagine how many ways it can kill you."

- Arjun Roldan-Leach, member of the Mountain Dogs.

Elder Beast (Theen: Trehé amasae Atra, "Animal, child of Hunger"; Torugk: Glugthle Kreuk, "Great Old Beast") is a designation to describe a collective of organisms of extreme power and bizarre physiology to the point of breaking the world around them with their mere presence. Each Elder Beast is a completely unique organism. While it is unknown how related, if at all, they are to each other, they all share the ability to retreat into a core upon severe injury or possible death. After a period of time spent in the core, the Elder Beast will regenerate a new body, similar but not identical to its previous one.
While some Elder Beasts are docile and will only attack when provoked, others wander around mindlessly trampling anything in their path, while some will seek out lifeforms to hunt or things to destroy. Regardless of an individual Beast's behavior, they are the most dangerous known organisms in the world and should only be approached with preparations to quickly flee or beat them into their cores. Do not take them lightly.

Basic Information

Anatomy

How alive the Elder Beasts are has been debated by eldritch biologists. While some are able to change certain traits on a whim, their bodies overall have some consistent order; they respond to stimuli; and they seem to grow in the sense they can regenerate their bodies upon reemerging from their core. However, they do not reproduce, unless one counts the Beast that emerges from the core as a separate organism from its previous form - and while behavioral studies of the Elder Beasts are naturally difficult, long-term observation of them suggests that they do keep at least some of their memories upon regeneration. Furthermore, some Elder Beasts clearly devour things, either organic or inorganic material depending on the individual, to provide themselves with energy, but others don't. The most widely-accepted theory among eldritch biologists is that these Beasts are sustained by a natural connection to the quintessence flow, but getting proof either way has been unsuccessful.
Some scientists have suggested that rather than natural organisms, the Elder Beasts are semi-organic constructs. Since historians and archeologists have found no record of their existence before the Nightfall, one theory is that they were weapons built for the Nightfall War, possibly by the Faceless Monarch.

Biological Traits

All Elder Beasts are large in size. The smallest documented Beast, the Osseous, stands at about eight feet high at the shoulder and thirty feet from nose to tail in its most recent body. The dimensions of the largest known Beast, the Sunken, are difficult to determine, as most of it stays submerged beneath the Lunar Sea with only its shell above the water, but it is large enough that sea nomads have used its shell as a place of rest in an emergency.
While each Elder Beast is unique in its true form, they are all identical while in their cores. The core is a perfectly smooth orb, white and phosphorescent in certain lights, with a diameter of about one foot and weighing roughly seven pounds. The core is impervious to any and all damage, withstanding crushing, burning, freezing, chopping, explosions, and whatever else researchers and irate adventurers can inflict on them. As Elder Beasts only regenerate out in the wild, the cores can be contained, but inevitably, some twist of fate will inevitably lead to it being lost from custody. The general consensus is that once a Elder Beast has been reduced to its core, the safest course of action is to hurl it into the void and hope that its trip through the endless expanse of nothingness will keep it occupied for a while. 
After an Elder Beast retreats to its core, most pieces of its body, even those that were separated, will disappear, presumably absorbed into the core as well. Only very small pieces such as rubbed-off skin or scales, as well as a few tooth or claw chips, may remain, likely because they are so inconsequential that the Beast does not need them to generate a new body. As such, significant remains cannot be taken for study or trophies.

Additional Information

Social Structure

Elder Beasts are solitary creatures. It is very rare that two of them will ever encounter, but if they do, they will fight until one is injured and retreats, though usually not to the point of returning to their core. Elder Beasts can be baited into encountering each other but, while this is (in the words of surviving witnesses) "extremely awesome", it is also absurdly destructive and dangerous to anything nearby, making it impractical as a method of warfare or entertainment. 
The largest known clash between Elder Beasts occurred in the Northern Wastes, leading to the creation of the aptly-named Titans' Crater. Fortunately, this was in an area where nobody was living, but was still so destructive that the myulins of the Lantern Woods and dragons of the Roosts joined forces to break it up before the damage spread to inhabited regions.

Lifespan

Indefinite, as they do not appear to be able to die permanently.