The League is a community built by Aaron Waponry and Patrick Skychaser. The Powered People that joined this League must swear that they will use the power in the good side of the law and stop the criminals.
"I, a Powered People, would like to join in the League. I shall use the Powers under my posession for the good deeds of people's sake, or else, I shall lose my powers."
The swear is tied by the Golden Rope, a gadget that Aaron found on a meteorite crash which apparently could lock swears. Once the swear is tied in, when they broke the promise, the consequences that they promised will be executed. Or if there were no consequences told, the punishment is simply death.