Extra Skills:

Extra Skills are a category of skills with abilities too high or too strange to be considered a member of the other 10 skill categories. Due to that, it is too difficult to divide the Extra Skills into sub-categories, except for one special group.

They also have a high rarity and many, but not all, are rewards for especially difficult trials such as certain Legacy Quests or Dungeons of Chaos.

These types of skills have a level cap of 10 instead of 50 like with other skills, but a single Lv.1 Extra Skill can have more worth than several other skills at maxed levels.

Some skills do not follow this trend. The most famous example of this is the [Apotheosis] skill. This skill does not grant any effects at all, nor is it any rare skill either, as most Celestials are born with this skill. 

Some normal skills may result in Extra Skills when one evolves them many enough times, such as [Sword God's Mastery] that originates from the [Swordsmanship]. These skills are considered the zenith of the art related to the origin skill and the goal of almost any practitioner of the origin skill.

Named Skills:

Named Skills are special skills even amongst the Extra Skills. They are based on heroes A-Rank and above creatures that live on Terra Sol or other worlds.

The effects of the Named Skills are based on the hero or creature in question, granting the skill owner the same abilities as that hero or monster.

These Named Skills can again be divided into Heroic Skills for the ones named after heroes and Monstrosity Skills for the creature variants. Some Monstrosity Skills that are based on other world's divinities and are therefore called Deity Skills.

Strengths and Weaknesses:

Needless to say, Extra Skills have more potential than other skills, even if they are not an improved version of normal skills. However, the effects of Extra Skills can sometimes be too much for someone that has not trained their bodies for those skills.

Take [Sword God's Mastery] as an example: It allows the user to perform literal miracles with the sword. However, what would happen if the skill owner's body had not trained with the many stages of [Swordsmanship] before reaching [Sword God's Mastery]?

They would break. The massive load on their bodies would be too much for the inexperienced swordsman, leading to more than a few broken bones at the best.

This holds even for those that have worked on their own to reach [Sword God's Mastery]. The reason why is that Extra Skills do not contain any form for limiters. Normal skills that consume mana would misfire if the skill owner lacked MP, but an Extra Skill would drain the owner's life force to compensate for the lack of mana.

In all, Extra Skills are skills for masters to obtain, that only masters can use. It does not matter how strong an Extra Skill is; if its user is trash, then so will the skill become.

Gust's comment: To think it would end things with a warning...

Garami's comment: Rather than that, explain how you're supposed to beat those that have those "master skills"!!