Quote Originally Posted by Petrocorus View Post
Level doesn't directly translate into CR.
You probably meant to say level adjustment doesn't translate into CR, which is true. Because levels in any PC class do translate into CR on 1:1 basis. So we are still looking at 21+ from character levels and 2 more from template.

As usual that doesn't mean that any creature with CR of 23 is incredibly hard for the party as presented, but power and defences of Xykon has been already addressed above without reference to his CR.