Magica, Elementa, Fate and The Universe:
Magica, The entire Universe exists because of it. It is what brings life together. Everyone has Magica inside them this is called "Magica Potential", although Reapers were the first to reach their Potential and unlock their Elementa, the first humans to reach their Potential were a group of monks, who mastered themselves, their souls and their spirits. Magica grows with the user, working on a scale of 1 to 10, with naturally 10 being the strongest. It grows and gets stronger in several ways, the main way being through raw training, exercise and sparring. There are only 4 beings in The Universe with a Magica level of 10 they are, Azrael The Heaven-Sent Spear, Fate, God and One who shall not be named. In general not many can break past a Magica Level of 6, because of sheer amount of training and stress it can put on you. Reaper's Magica Levels grow as they fight and naturally adept, The average Magica Level of a Reaper is between 2 – 4.
Elementa is a physical manifestation of one's Magica, taking on unique and different forms, with different abilities and levels of power. No Elementa is perfect, everything has a counter Elementa and that counter Elementa has a counter. Even if your Magica Potential is reached it may take weeks, months or even years for you to unlock your Elementa ex. Sam's Grimm Fire didn't unlock until 2 years after he reached his Magica Potential. And before you ask just because Elementa has "Element" in it doesn't mean all Elementas are based on Elements ex. Carter and his Elementa literally being a Gun.
Fate, the writer of The Universe she is the 2nd oldest being in existence. Her only duty is to write the story of Humanity. The warriors of Fate serve her and follow out her orders, taking care of any of those who dare to oppose her. She has no Good or Bad alignment, but rather her alignment is in "Balance". She had a small relationship with Azrael, but it didn't work out. It is unknown what her plan is with Sam Reyes. She also has a younger Brother named "Destiny" and a younger sister "Time". Fate, Time and Destiny.
The Universe, Simple to understand, it was created by God. In the universe there are infinite "Dimensions", with different planets in them, only one Dimension has Earth in it. Basically put it "Each Dimension is unique and different". Each Dimension is far away from the other.