Legend, the last rank within the skill and ability ranking system. That being said, those who are at this level are often powerful enough that gives them their title. This is preceded by the Hero/Heroic rank.
Legendary abilities are the most dangerous, the most powerful and complex types of abilities. That being said, they're also to consume a number of energy so these types of techniques are often meant to be used once and only once given their dangerous capability. Given these abilities are complex they are, it should be noted that they're often unique to the user's themselves.