Used as a material for some items.
Can be gathered from bosses with >=50K HP if you have Extractor in inventory.
A reference to Undertale's souls of the creatures inside. Upon killing an enemy, their soul shatters, but if the Extractor is used the soul can be contained. Human souls are different, as they are much more powerful than normal monster souls, and can last a short period of time before deteriorating without additional help from soul containers or similar machinery.