i think extra history got it a little wrong,it seems they got confused in times kavad escaped and got aided by Hephthalites. as far as i remember kavad got captured by Hephthalites in his father third invasion. anyhow about your questions we do not have much data on Fortress of Oblivion however the reason for the name is indeed interesting it was called Oblivion because the purpose was to forget the people who were prisoners in there.now if someone was in this prison it was forbidden to mention their name,if you did however mention their name you would be executed or made a nice spot for yourself in the prison right next to the guy you mentioned. now like i said before i think extra history got it wrong,kavad was a prisoner in there only once and that was after the mazdak thing. now about how he escaped?he had a little help from his sister or wife. his sister came to visit him but by a plan to make him free,in the room that the two visited there was a carpet now kavad hid in the carpet and a man that was with his sister took the carpet that now held kavad and put it on his shoulder and with his sister went to get out of prison,now they had to pass the guards and when one of them asked why you are taking out the carpet with you? his sister said that i bleed on that carpet and because of that he is taking it out.when the guards heard this they didn't even get close to investigate and this is because of a religious reason which you can google it later and if i remember correctly the guard also liked kavad's sister a little bit so he let her get out easily.

it was not easy to escape from this prison but surely you should know that its also not that easy to hold a great king in any prison,is it?

i hope my memory served me well in answering your questions.