Skills have similar grades to treasures. Except, minimum grade starts at common rank.
1. Common grade - corresponding to D rank
2. Rare grade - C rank
3. Epic grade - B rank
4. Legendary grade - A rank
5. Mythic grade - S rank ( Demigod )
(There is no unique grade among skills.)
Though skills have no requirements for learning them, they need a certain amount of mana to be used - known as skill cost.
Note ;
Maximum amount of mana a rankless individual can have is 100. ( Mana = intelligence × 10 = 10×10 = 100 )
And even a weak common grade skill like fireball has a skill cost of 50 mana. A level 10 individual can only use it twice before running out of mana.
For some powerful common grade skills, the cost can be as high as 200 mana . { that makes it almost impossible for low rank individuals to use it ( except in cases of someone possessing a mana raising treasure )}
So it's usually rare for low rank individuals to use high rank skills.