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
「[自]{みずか}らを[表]{あらわ}す[呪文]{じゅもん}に、自らを[律]{りっ}する[韻]{いん}を[持]{も}たせた[英雄]{えいゆう}。」
As is often the case in Japanese phrases and sentences, the key words come in kana and they are very short.
「A + に + B + を + [持]{も}たせる」
=
"to let A have B"
「持たせる」 is the causative form. You cannot use 「持った」 in this phrase with both に and を.
"A hero who let his self-expressing spell have rhymes to discipline himself."
「自らを律する韻」 is, by no means, a common phrase, so more context (lots of it, perhaps) would be needed for a better translation.
That fan-TL is a joke; you know much better than him/her.
