Demons so powerful that they can smite balors to dust, and bring cities to ruin.
And are more threatening than Demogorgon.
Seriously, In the Fiendish Codex, Big D is CR 23; and in the Fiend Folio, a Klurichir is CR 25.
But, in a table listing demons by CR in the back of the Codex, a Klurichir is CR 15.
So is the Fiend Folio wrong, or is the Fiendish Codex?