Once Upon A Time
There exists a skeleton, a weird skeleton. He kept on going different worlds, different universes and such, making enemies left and right for god knows why. One day, he got bored of going through his usual travels and decided to stay at his place while messing around with the souvenirs he got from them.
One such souvenir was a Determination Soul, where he got it from defeating his enemies in his travels. Unexpectedly, he happened to have quite alot of them, so he fused every single soul into one, making the soul that you know today, the second vessel; Serenity.