Underneath the castle proper, there is a dungeon, and in that dungeon will always be the Beast.
Every so often, the lord of the castle descends black stone steps to the dungeon. In front of him are iron bars, sturdy and unbreakable. And past the iron bars is the Beast.
The Beast rages and tears at its fur. It stomps until the dank stonework beneath its hoof fairly trembles. It roars and howls in the agony of its madness. There is no need for a torch; its brilliant red eyes glow with hate enough to light its cell.
The lord of the castle regards the Beast with frustration. He has tried to tame it in the past, but it resists all blandishments, ignores all entreaties. And in these times when he descends to confront it, his anger grows. He tears his hair out. He roars at the Beast. He shakes the bars, but futilely, for he himself installed them and knows their strength. He screams at the Beast, denounces and disowns it. He rips off his own skin, revealing matted fur beneath. He kicks off his boots, revealing hooves. His newly-found claws tear away his face, revealing the snouted animalistic visage underneath.
This display is enough to quell the raging spirit of the Beast. The creature becomes more and more recognizably human as it witnesses the torments of its jailer. And while the once-proud lord of the castle shrieks at the Beast, the creature dresses itself in prepared garments and ascends the black stone steps behind it, to become lord of the castle again.