The process of a soul being created is poorly understood. There are energies in the brains of material creatures that certainly lead one to believe that brains have something to do with it, though the ability for a soul to exist without a brain and without those energies simply raises further questions. For quite some time, it was believed that the one mother created and maintained souls, and the energies in brains were simply the method of the soul communicating with the body - however, her absence for the past 5 decades has proven that she is not required for soul creation, though she may still have something to do with it.
A century ago saw a rise in experimentations with souls. Biomancers and other wizards with metaphysical interests would create brains without bodies and study the formation of their soul. The souls formed were usually irrational and lacked astral bodies, which led the experimenters to believe that souls required physical senses on top of brains to form correctly. The cases of conjoined twins each having their own individual soul (one for each brain) caused a trend in biomancy for multi-headed creatures, to see if something with multiple brains could have just one soul (such as the hydra or chimera), or to see how many souls they could cram into one physical body.
One poorly conceived experiment along these lines is the monster now known as a "soul eater." Soul eaters were originally much smaller, though reproduction over time has caused them to grow much larger since they've escaped into the wild. Modern soul eaters are ten feet tall, reptilian humanoids with long arms, short, stocky and powerful legs, and a head the size of their torso with a maw that looks more like it belongs on a bullfrog rather than a 10 foot tall tower of meat. Soul eaters also have a thick, warty carapace on their back that is perhaps their most distinguishing physical characteristic.
Soul eaters tend to swallow their prey whole. When they do, their unique physiology preserves their meal's brain and carries it to the carapace, where it is added to a cluster of neural tissue with enough bulk to drown a horse. This process, while traumatic, manages to maintain the bond between the soul and the brain. Thus, a soul eater has in its carapace the brains of every meal it's very had - and by extension, traps their souls within itself.
This neural tissue is not connected to any sensory organs and cannot control any muscles of the soul eater, so we can only speculate as to what the brains trapped within the neutral tissue experience. Creatures that are aware of the inbetween can often see astral hands and faces trying to claw their way out of the soul eater's carapace. Even entire astral bodies connected to the soul eater by chains have been reported. The sounds the astral bodies make have been said to be cacophonous and horrifying.
Soul eaters have no life expectancy, and they breed by budding. When the number of brains stored in the carapace reaches a critical mass, the carapace buds - and 5-20 new soul eaters will grow out of it and then go on their merry way, each carrying a percentage of the brains and souls the parent had trapped within itself. The only known way of freeing the souls trapped within a soul eater is to kill the beast.