If 'better the devil you know,' they say, then tell me, Anna... what do you know of the Devil?

Nothing, yet. You would need to find someone more knowing. He is in the cell behind the sick troll.

It is the Wizard of Wunderhorn! Ask him about the Devil.

That story contains the elements you need to solve the riddle.

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...

First hint:

A sight most horrifying an indescribable beast, gorging itself on... unidentifiable hunks of meat.

There's piles of meat in the Wizard's cell. Take one from there. Give it to the Devil.

Second hint:

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.

In his true form! The riddle is solved.