Long ago magical and mythical creatures walked this earth, swam in its seas and flew in its sky. They were countless in types but limited in number. As humans began to spread like an unstoppable virus they came into conflict with these magical creatures and people.
So to avoid war, the humans that had been gifted with Magic themselves decided to remove the creatures from the world. Using ancient spells, they began to turn those mythical people into normal humans.
Through magical means, dragons were turned into men, mermaids into women and so on until only humans remains. These people remembered their old lives, and some tried to return to their old bodies but without magic they where helpless and in time they moved on and their child and grand child where as human as anyone else.
The human wizard and witches who did this to the world knew that magic could not be destroyed only contained. So after each creature was transformed a new magical shard was made containing the magic native to that creature. Those shards some small enough to fit on a ring, some large enough to take up a whole wagon where place carefully away in vaults around the world.
For if they would be broken the magic would be released and any human within a certain range would become the creature whose magic was contained within. Even if a pure human broke a shard, they would transform into said magic creature and in a few cases the human changed not only into a magical creature but changed gender and race as well. Those humans transformed by a shard did gain the creatures magic but not the memories of those creatures.
The size of the shard set the range of those who would be transformed, a small ring size shard may transform 1 or 2 people into creatures but one the size of a wagon could transform a whole city.
These shards where locked away and forgot about but now thanks to treasure hunters and fools who believe magic is just a myth, they are now free on the world.