There is a character (The hero) who uses a spell and this is the comment another character does on that spell.
…自らを表す呪文に、自らを律する韻を持たせた英雄。
My translation
A hero who had a spell which represents him, a rhyme with which he disciplines himself.
Fantraslation
A hero who used a poem about himself as his spell.
How come it's not 持った but 持たせた?
What's the difference here?
持たせる=To have someone hold something; To allow someone to keep something