Legendary is subjective. He is one step from being a Law Master. For perspective he is one step from being a pope. Highly revered and respected across the land. And there is only one artifact, which he holds, thought to be held by Alathia herself. He has a duty and capability to enforce his rulings and keep the peace. He’s a walking judge, juror, and police force. Only he and his like can use magic. When he approaches, everyone knows him; everyone welcomes him before he starts working.  In his world, he is of legendary status. 

He can do buffs without consent. He can do debuffs without consent. He can only do trade off or huge buffs with consent. He can only do huge debuffs with consent mainly the enemy’s. The farther the divide between power as observed the more likely he’d need something to justify it, consent being the easiest. The artifact is considered to be like an eye of Alathia, so she is always watching that he’s not being unjust. 

The buffs seem highly limited but theoretically as long as both sides of equal power he could make everyone as strong as a baby (realistically though I would never get away with that). The power of buffs and debuffs are also tied to the person enchanted and their skill to use them. A person who never loses if they are really close can easily take advantage of a spell that make him faster or the other slower or both. 

He can also cast “rules”, such as deny the enemy the high ground by making it a rule not to be there, but I figure that would need more finesse. He’d be influencing the game field, not the game. His “rules” are enchantments. He cannot change the rules of the challenge like a dragon nobility can, nor can he hinder or prevent the opponent from completing the objective. 

The scale he can apply these isn’t that tight either. The main point of the limits is to have a story reason why he can’t just make everyone OP while tying it onto his character. 

Hopefully that helps clear things. In the end he gives buffs and debuffs so that his side can have a better shot at winning.