Olwen's bait theory was starting to seem like the most obvious conclusion as Arthur headed up the group of tightly formed knights, holding his sword in his right hand, and Merlin's shirt collar in the other, keeping his servant partially in front of him. They went down the path a little way and the creatures, on seeing them, started to skitter upwards. Arthur felt Merlin quail a little and gave him a shake. Merlin watched the creatures running up the slope and his eyes swept the beasts.

He could feel the tension of the people behind him. They were all breathing heavily, trying to control their fear. Arthur had told them nothing, except to never tell anyone what they would see. Merlin blinked, something in him suddenly steadying. There was no doubt in his mind he knew he could do what Arthur required of him.

It was just doing it. Merlin had wondered, before it had happened, what it would be like to reveal his nature to Arthur. It had hardly been the easiest thing, nor had it been as hard as his darkest thoughts. Arthur had just been human about it. Hiding his head a little, being angry, taking it out on Merlin. Merlin fought none of it, thankful for his life, surprised that Arthur tried to find some measure of understanding about it.

Merlin wondered if Arthur hadn't taken that understanding a little too far. There was something very naïve about his belief that the knights would keep to that vow. Merlin felt differently about it. One of them would think that their commitment to Camelot and its King would negate the vow completely, and Merlin could do something about it. He could do nothing, he could let the serkets kill them, blaming his lack of power and just keep himself and Arthur alive.

Then living with himself would be something of a problem. He would have sent five good people to their death and that was not what his gift was there for. Of all the things Merlin had been forced to deal with, and act on, that fact would remain. If he could save people, people that had no right to die, then he could not stand by and leave them helpless.

"Merlin?" Arthur growled.

The creatures were starting to run close. Arthur's hand tightened on his shirt, and the other swung his sword, nervously, as Arthur made himself ready. The tension vibrated off him, and he shuffled his stance a little, staying close to Merlin, brushing against him as Merlin waited. Merlin lowered his head, gaze locking on what he needed to see.

"Just wait," he said.

"What for?" Arthur asked. Merlin felt the material of his shirt twist in Arthur's grip. It wasn't to hold him in front of him, as some of the knights had assumed. Olwen was having a horrific idea that Arthur was going to pitch Merlin into the oncoming beasts as a distraction. Nor was it to stop Merlin from running away, as Gareth was starting to think. It was simply to prevent his very – and it always happened at the wrong moment - clumsy servant from falling flat on his face.

"Maximum impact," Merlin growled.

The panic had long ago fallen away and something else built. The wave rose up in him. Merlin had never really noticed it before, but in Camelot the control he had to put on himself made him realise he was doing it, and now he disregarded it. He exhaled, dropping his shoulders and raising his right hand. Next to him Arthur released a breath, as he always did just before the first strike in a battle, to calm himself, to release any emotion that could distract him.

Merlin kept the emotion, his anger channelling, thinking about whoever had put them here, herding them in like cattle.

"Merlin?" Arthur's voice sounded in his ear again, this time a little uncertain, demanding help.

Taking a deep breath Merlin unleashed the building spell, roaring the words out, throwing them out from the depth of his being. The shockwave surged down the mountain, he felt his power uncoil and lash out. The scurrying beasts were suddenly airborne, thrown across the rocky wasteland of the mountain, bodies smashing as they landed. Some managed to avoid the worst of Merlin's attack and then scrabbled over the bodies of their comrades. Arthur almost started to run, and then Merlin's arm whipped back to stop him.

"Wait!" Merlin ordered.

Olwen stopped staring at Merlin as if he had grown another head and looked down as he felt the earth vibrate underneath him. Small stones were dancing erratically and the knights looked around, and despite the shock of what they had seen they pulled closer to Merlin, tightening up against the wave of magic around them.

Arthur looked at Merlin. He was fully focussed on what he was doing and Arthur felt a shiver of fear as he gazed at Merlin's profile. It was his eyes, not just the gold flashes in them but the fixed stare that made a shiver run down his spine. Merlin was no longer his hapless, stupid servant, a role he had continued even after Arthur knew what he was. This was something else, this was what Merlin could be, and Arthur wondered if he hadn't done something terrible to him.

"Merlin?" Arthur asked.

"What the hell is that?" Olwen said.

Merlin seemed to snap out of his trance and he turned to look at Arthur. He smiled as best he could, but the guilt and fear had returned to Merlin's face. Arthur smiled back as best he could, but he could not hide the fact that Merlin's sudden changes were unnerving.

"Just something else," Merlin said.

"They're coming back!" Robert shouted.

Merlin's eyes changed again. He started to chant in a low tone. Arthur couldn't hear a clear word in the low murmur Merlin was using. Arthur didn't understand what he was saying. The rumbling beneath their feet started to get louder and he jumped back a little as the ground a few metres in front of them started to crack. A few of the knights yelped in shock, but it was drowned out as fire raged out of the gap. The thick red hot liquid spewed out as Merlin tapped into the remains of the volcano that had once been the mountain, and sent it surging downwards.

It cleared the pathway and Arthur saw that part of the plan.

"Go!" he yelled, jumping forward and bracing his arm to keep his grip on Merlin.

Merlin ran with him, his body reacting automatically and he trusted Arthur to help him keep his feet under him. Arthur was bracing his arm every time he felt Merlin's feet slither, not just because he was always so uncoordinated but because his mind was on something else. They could feel the heat raging from the lava but it never burnt them. Arthur skidded as one of the beasts tried to pull free of the liquid. Lashing out with his sword he cut through one of the legs. The beast's screams increased in pitch. They were all shrieking and writhing.

As they reached the base of the mountain another serket tried to lash out. Olwen and Robert hacked at it with their swords, driving it back. Then they ran on, reaching the woods. Arthur released Merlin, pushing him on and yelling at the knights to keep going. In the shadow of the tree line Merlin and Arthur paused, turning around to look at the devastation that Merlin had wreaked. They only paused for a moment before they glanced at each other and ran on.

They only stopped again as they crested a rise and reached safety. All of them paused. Arthur dropped his shoulder against a nearby tree and he gasped for breath. Merlin, on the other side, put his hands against it, taking deep breaths, and he felt sweat drip from his face, and down his body. Around them the knights sprawled gasping for breath. But the recovery came and they straightened up, and looked around.

Eyes flashed around, avoiding gazes, drifting to Merlin, to Arthur, to each other. Merlin glanced around and saw what was on their faces as they realised why Arthur had extracted the vow from them. Arthur Pendragon was harbouring a sorcerer under his father's nose. A sorcerer that had saved their lives, and who had unleashed a power that they had all felt, as well as seen.

Arthur took several deep breaths and stepped away from the tree.

"We need to get back to the convoy," he announced, jogging in that general direction. Merlin looked at him and then the five pairs of eyes that stared at him.

"Merlin!" Arthur's voice yelled back at him.

Merlin turned and ran after him, stumbling over a tree root as he ran to catch up with Arthur.