The Urarius has no shape of its own, and as such requires a host to survive. It loses its strength and fades away if unable to feed on souls, and as such is always hunting for more.
It chooses a large and heavy animal to possess, most commonly a kind of black polar bear weak to its influence. It will pull in its victim's soul using its mouth with a very strong pull, as though inhaling. Even if the bear is killed, the Urarius continues to puppet its corpse (although without the bear's natural guidance, its movements become jerky and difficult) until the body is in too terrible condition to continue moving.
This kind of spirit is quite rare, and can only be vanquished with an exorcism or any other spell or treatment that would remove a spirit from its host.
As soon as it has found a host, the Urarius takes absolute control. It will eat the souls of anyone nearby, which has caused the black polar bears to spread out and live alone as much as possible. The relationship the two share will last for the rest of the bear's life, and even past it.
The bear feeds on the same souls as its controller, so it stops eating or drinking. It also becomes much more docile, ceasing to hunt or attack others or use its claws and fangs. It instead attacks when attacked, using its large paws or body to ram the victim. It will also use its heavy stature to pin the victim down in order to take their soul.
Even if the bear is killed, the Urarius will continue to possess it and make it move, keeping the corpse fresh for as long as possible with the energy of souls it gathers. Even if the bear's brain and heart are removed it will continue to move the body, only leaving its host voluntarily if the bear's head or thick legs are severed and it is rendered incapable of moving or feeding.
It most commonly appears as a large, heavy polar bear with black fur. It moves naturally like a bear would, though it does not growl or roar and is unusually docile. After death, it no longer moves naturally and its movements are jerky and strange, like those of a puppet on strings, and its wounds keeping a fresh look.
If you are in a group, trying to resist its soul-stealing capabilities is actually more dangerous than letting it do as it wishes. Someone else can break its focus, but trying to pull free on your own can get your soul damaged or even torn in half in the ensuing tug-o-war.
Only one of these beasts has ever been in play, due to its dangerous nature and difficulty to kill.