Arthur Pendragon had based his whole life on the idea of bravery.
From an early aged he had been schooled in the art of warfare, trained to show no fear in front of the men he might one day lead in to battle. As the prince of Camelot he had to be a rock steady warrior, a figure head for the whole Kingdom
Many of Arthur's most famous deeds, his most heroic acts were actually preformed out of fear. Not of dying or of the foe he faced, no what Arthur feared most was his father. Uther did not tolerate weakness and Arthur knew it, and for as long he could remember his whole life, his whole world even had revolved around pleasing his father, showing no cowardice, no fear.
When alone, late at night in his chambers when no one could see he became wracked with doubt. Fear of disappointing his father, of being unable to rule and protect his kingdom flooded his mind and soul. Sometimes, but only sometimes, he let these worried totally consume him. His body would shake with the force of his tears and eventually he would fall in to an exhausted sleep plagued by visions of Camelot falling. In the morning he would rise and redouble his efforts to please his father.
One look was all it took though in order to make Arthur disobey his father, just one look at the pale raven haired boy who had willing swallowed poison in his place. His fear of his father was nothing compared to the fear that gripped Arthur when he looked at Merlin clearly in pain. The thought of going back to a life without him, of never seeing that goofy grin, of never being called an ass, of all the things they would not do together.
Arthur realised in that moment what real fear really was.