Hey there are some things we need to know,
The Level of Existence,
Every being have born in world have Energy of world depend upon their existence it have different name.For human it is called Life Energy holding in their body, For spirits it's Spiritual Energy, etc. this Energy Strengthen one self, creating core inside body and increase their limit of energy Capacity,
Depend upon this there are Rank of existence in beings and Every Rank have 10 level:
Notice : Level of Existence different from Magic Level and Level of Arts one can use so even a High level Existence does mean they are strong and can do, but not necessary they can do.
Rank 0 (Level 1-9) : It is called Basic Level
Normal human and heavy working people, the lowest level monsters, this including apprentice warrior and mages who can use First And Second level Arts or Spell
Rank 1 (Level 10-19) : It is level of Warrior.
Strong human, Trained warrior and low level Mercenaries, people above Normal level who can do Third level Arts or Spell, the level of Low level Spirit and Normal monsters
Rank 2(Level 20-29): This is Peak Warrior Level.
The highest Rank that a Normal person can obtain, equal to basic Spirit level, can use Fourth Rank Arts or Spell, look like genius in small village
Rank 3 (Level 30-39): This is Master Warrior Level Rank.
The Rank of Master according to commoners above normal people and can fight against Normal Spirit and do superhuman things, far beyond level of normal human, can do Fifth and Sixth level Arts or Magic, the peak of human understanding and will
Rank 4 (Level 40-49): This is Hero Level.
The Rank of Genius that born with potential above human understanding, a Rank can't get by just hardwork, can be equal to young dragons and weaken Greater Spirit, which is above normal Spirit, People on this Rank can do Seventh Rank Arts or Magic
Rank 5(Level 50-59): This is Great Warrior Level.
The highest Rank that a mortal being or a person can obtain because it is a level where the limit of Human body stop and one need to be more than a being made of Flesh and Blood to go beyond it, the Need of Power Like Nature, the level of Great Spirit and a level that a Normal Dragon ( Not young, Not Old, Nearly 100 years which grow with Age)
Rank 6(Level 60-69): This is Peak Human Level.
The highest level a human with all of his level can gain, the highest level a human can have with it's body, after which the human body collapse on it's own without external help. The level of Dragon Lord and can do Eigth and Ninth Level Arts or Magic that can resist and shake the basic human understanding, A disaster.
Rank 7(Level 70-79): The Greater Human Level.
It is a level where mortal limitations start breaking, normal attacks stop working and have high resistance to all things, beings can move faster as sound and shake the surroundings with sheer power. A people of this caliber can use highest level Magic : Absolute, Perfect and True Magic. that can shake the whole kingdom, but only if they solely focus on it.
Rank 8(Level 80-89) : The Nature Level
A level where only the highest level beings can exist and have power like force of nature. Like the Ancient Dragon Lord, Primal Spirits, Natural Catastrophe, that can break the balance of power and destroy Countries on their own, The energy they release through body can kill any normal person instantly, even a Rank 2 or 3 being can't scratch no matter what they do.
They start getting resistance to Elements, Space Manipulation, Time Manipulation, Death Resistance, Mental Resistance, Physical Resistance and Resistance to law. They have Resistance to all types of attack to some extent. They are not immortal but hard to kill without overwhelming power.
Primal Spirit : The highest level Spirit that can absorb their respective element and grow infinitely, can't hurt by same Element instead use it to heal and Strenghten, it is right to say, they are the Element.
Rank 9 (Level 90-95): The Divine Level
A level where one can become the Master of a Power, a law that they can control like a God who can control and manipulate a force or a law of nature, They get immunity to Instant Death, can basically resist laws which are like absolute for mortal human.
Rank 10(Level 96-100) : The Perfect Level
A level, what a being go at maximum limit, that a Normal being without the help of world exist, A being who stand against world and can shake the world but without the help of world, infact against the world, their power can break the laws and can make impossible into possible. They can break the Reality and Kill the immortal, Can turn Sea into Molten Lava land and also turn Deserts into Big Green Forest,
They can't destroy or change the laws of world but they can bend it so much that one can question it's existence.
Rank 11(Level 100+): The World Beings
Beings that are personification of laws, they are don't have a definition of power but they are Disasters born with powers that are equal to laws and support by laws of world to destroy the balance and recreation.
There are proof of their existence in Ancient books but there are no one who see and there is no indication of their existence now, no one see only once seen 2000 years before.
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Monster and Humans have Energy cores which created by them throughout hard work and continues training while going on a path.
These cores store power and Strengthen the user, let them use powers above Rank 6 because Normal human can't grow beyond Rank 3.
Any being can have 3 cores including humans, the cores are present in lower body, Heart ( Middle Body)and in mind (Upper Body) place which also determine user's speciality.
A person can have more than one core but most beings unable to limit with which they are born or solid foundation of other core make them unable to form another one.
Lower Body : Physical Fighter with Strengthen type, have extremely pure, stable and dense energy purify by body, best for close quarter physical fight but it can also be use for long range Magic and Aura based attack but it will be best for close quarter physical fight.
Middle Body : A core near heart can combined Energy and emotions, explosive power, very violent and can easily use with weapon and armors in form of Aura, this core is best for Aura user.
Upper Body: A core near Mind combined mental energy with it, make it extremely versatile and bend according to user's will.
Cores are mostly immaterial form inside a being in form of energy and can also be materialized but it need huge amount of effort,
A human need to materialized their core to go above Rank 7 level.
Monsters and Nobel Rank Human born with cores due to their lineage can use the energy make them stronger than normal humans, some Nobels human born with Multiple cores.
It is basic knowledge of Existence, I hope you like it.