In The Silmarillion and this post, it is mentioned that the universe was sung into existence by the Iluvatar and the Ainur. And that some of the Ainur integrated themselves inside said universe and created the Valar and Maiar.
What happened to the other Ainur who did NOT integrate themselves into the universe? Were they silent spectators? And what were the names of the ones that did integrate themselves?
Also towards the end of the post, Buzz mentioned that "Fëanor will finally repent his crimes and be released from the Halls of Mandos" so that the two trees can be resurrected once more.
What were Feanor's crimes?