"The Fomorroh will suck the life force out of you. Everything that makes you Merlin will be gone. And in its place there will be just one thought. One thought that will grow until it's consumed you completely. One thought that will be your life's work"

Merlin mulled over Morgana's words, one of the last things he remembered before his mind had been taken from him. He rubbed the back of his neck subconsciously, remembering the squirming serpent underneath. He was glad it was gone: he couldn't have lived with himself if he had killed Arthur.

In a way, though, Merlin had his own Fomorroh: Destiny.

Ever since he had learnt of his destiny to help the Once and Future King unite the lands of Albion, Merlin had dedicated himself entirely to fulfilling it. It had grown from an obligation to the dragon to save Arthur (God, he had been such an ass back then) to his purpose in life. He had sacrificed so much to make it happen.

One thought that had become his life's work.

Protect Arthur.

So yeah, I was watching 'A servant of Two Masters' and noticed how Merlin's drive to kill Arthur was very similar to his drive to continually protect him. That he goes to such lengths to achieve his goal. What's disheartening though is that even though it consumed his mind, he still failed. I really hope this wasn't foreshadowing for the finale.