What to do at the end !? (The Witcher 3: Wild Hunt)

run to the house and there by the inoperative fountain break the wall

Biolit
You cannot even imagine how grateful I am to you!

Don't forget to pull the sword of the serpent school out of the stone

Fima51
sucks that on the new game + a regular sword is given and not a legendary one

What is the mystery?

If you free the cat and the dog by taking the rose, they give a hint on how to outwit the demon.

By the way, it is a pity that there are few such verbal riddles in the game. So that you have to solve something on your own, for example, to remove curses