Mortals are beings that only exist on the planet known as Cavus. Created when the planet aligned to a life leylines path of travel, ambient mana was sparked and became aware of its own existence. The mana was alive, it could feel, and it could think. It was a spark of energy that sought to continue its existence. In trying to maintain its existence it evolved into what mortals are today.
When the mortals started out, they all looked the same. They did not have access to the magic and mana that the birthless did, and some still don't. Small and frail, their sparks were easily washed out, and blown away. They learned to overcome this weakness by using the pockets of mana on Cavus, later called places of power. The mana would change them. It gave them strength. It altered their appearances. It made them unique to one another.
As time went on, Mortals began to interact with Birthless, and through their interactions, Mortals mimicked the Birthless going from an innocent or gauche creature to a more political and emotionally driven beings. What once was a need to survive, so that one could understand being alive, became understanding what life was, and how good it felt to be alive.
Each Mortal's time on Cavus was limited, as all sparks die out. Mortals distinguished themselves from Birthless with their emotions and their actions in this short time. The Birthless were fascinated by mortals, in some cases obsessed with peculiar facets of mortal culture and life. While it resembled their own, Mortals often took things to the extreme, as opposed to the ordinary. A Birthless would see that something was stolen from it, and would simply replace it. A Mortal would see that something was taken from it, and would embark on an adventure that could last their entire life in search of what was stolen. This wasn't always the case, it could be a number of things, and unless a Birthless had studied the mortal beforehand, it was near impossible to tell what would happen next in a Mortal's life.