It is the Wizard of Wunderhorn! Ask him about the Devil.
It is a story?
That story contains the elements you need to solve the riddle.
Tell it again
The story went like this: He has many guises, you see, but... few have looked upon his hideous, true self... A man I once met, he saw it. He was a thief who broke into a great and wealthy mansion... He then hid, in a cupboard, until the midnight hour... Just before the time for the theft was right, he saw it, through the keyhole... A sight most horrifying an indescribable beast, gorging itself on... unidentifiable hunks of meat. This thief... he shook with fear. Be cautious, dear child...
I can't make it
A sight most horrifying an indescribable beast, gorging itself on... unidentifiable hunks of meat.
Hunks of meat?
There's piles of meat in the Wizard's cell. Take one from there. Give it to the Devil.
Knock on the door and talk about the knowing riddle. Anna will give the meat.
Well, he is probably eating there...
Just before the time for the theft was right, he saw it, through the keyhole...
There is a keyhole in the Devil's door. Look through it by looking at the door.