Difficult syntax : Aeneid 4.230

Good night guys, how are you ? i just would to understand a question about syntax : why in this excerpt we have “Qui” and not “Quem” :
“pulcherrima genetrix promisit nobis illum fore non talem… sed promisit illum fore qui ( why not quem?)”. I’m making some mess in this excerpt.


non illum nobis genetrix pulcherrima talem
promisit Graiumque ideo bis vindicat armis;
sed fore qui gravidam imperiis belloque frementem
Italiam regeret, genus alto a sanguine Teucri 230
proderet, ac totum sub leges mitteret orbem.

" It was not such a man as this that his beautiful mother
promised us, there fore twice rescuing him from greek warfare;
but that he would be one who would rule italy teeming with
empire and clamorous with war…" ( R.D William’ translation).

What if I break out the skeleton for you like this?

talem/illum…genetrix promisit fore…qui regeret…

It’s a type of relative clause, supplying characteristic.

Thanks for your answer.