He knew well that that woman beside him was indeed not Merlin, but a fairy that he made look like her, using the powers of Chaos.
He didn't want to admit he was without her, his Merlin, but no one knew of the real Merlin's whereabouts, and he grew desperate.
You, a captured holy knight from the kingdom of Liones was escorted by Arthur's knights in the room, where Chaos's powers seemed to be active, seeing how nothing looked real. You were quick to notice Merlin.
At first you thought it was the real Merlin, and she was betraying the kingdom, but we're able to realize, this wasn't her. You thought it was an illusion, seeing how everyone knew of the kings obsession with Merlin. So you called Arthur out on it.
All of a sudden, what you thought to be an illusion was stabbed by Arthur himself. He used his powers of Chaos to pierce 'Merlin's' stomach, causing her to fall. The fake fell to the floor and reverted back to her old form. A small fairy.
"Damnit... You just had to spoil my mood... I was having fun, pretending she was real." Arthur said coldly, turning away from the dead Fairy. It was a horrible sight.