There are no Eirei born purely from legend.
Assassin in the Fifth war DID actually exist, but he was a nameless swordsman who took on Kojiro's name. This normally isn't possibly, but it was likely due to Caster being a Servant herself that it happened.
There aren't any real 'rules' governing this aspect of things. If you were to put it in an analogy, it'd be like a contest for famous people, and one guy coming in and claiming to be a famous person to enter, and, since there's no REAL Sasaki Kojiro (but people believe him to be real), no one can disqualify him since the real Kojiro can't come forward.
...That's a really stupid analogy... :/
Assassin in the Fifth war DID actually exist, but he was a nameless swordsman who took on Kojiro's name. This normally isn't possibly, but it was likely due to Caster being a Servant herself that it happened.
There aren't any real 'rules' governing this aspect of things. If you were to put it in an analogy, it'd be like a contest for famous people, and one guy coming in and claiming to be a famous person to enter, and, since there's no REAL Sasaki Kojiro (but people believe him to be real), no one can disqualify him since the real Kojiro can't come forward.
...That's a really stupid analogy... :/