It had slowly built up in his ever-hardening chest. Over weeks and weeks after Uther's death, the guilt and resentment had all piled together until it had become a twisted mass pushing against the edges of his heart. He was afraid if he did not let it out soon, it would push the little corner for the current King of Camelot out of his place…
But that ball of ugly emotions felt very permanent, like a pebble stuck between the hard castle stones that made up the walls. It was too deep inside to be able to scrounge out. The magic pulsing in his veins begged to be released, to be understood, but that was only a small part of what was making Merlin truly angry.
The day when his father, Balinor, passed away had come and gone recently. While Arthur had been able to mourn Uther, the man who lead to Merlin growing up without a father and indirectly taking Balinor away from him permanently, Merlin could never show his grief. He could never take the day off and walk to his father's resting place to lay flowers where his body had decayed. He could never bring himself to tell his mother that the love of her life had died while he had watched.
He could never tell Arthur that Balinor, the "last" of the Dragonlords, had been his father and damn it all, he deserved to cry over him just as much as Arthur did. He deserved a friend who would wait for him through the night as he buried his face in the dirt and let out the gut-wrenching yell he had always wanted to do. Scream into those mountains with a furious voice, asking why something so precious had been taken from him when he had only just found it.
There were times these days he saw Arthur and that black ball would swell and Merlin felt like showing Arhur what he was capable of, how he had done all those things for him. How he had saved bloody King Arthur's neck hundreds of times and how he had sacrificed ever being able to show his own grief, fears, dreams and wonders.
But then Arthur would turn around and smile, try to joke, or just be usual pompous Arthur with that hint of sobriety he had adapted these days after becoming the ruler of Camelot. That darkness would shrink, but it never went away. Sometimes Merlin could swear it was slowly growing, just waiting for Merlin to snap.
Merlin was afraid of that day, because then he knew Arthur would never even see the real him. He would only see and fear the darkness.