Note: Spoilers for episode 1x09 "Excalibur." AU. Character death for the person Tristan defeats here (if you've seen the episode, you'll know who this is).
When Tristan Dubois arrives at the gates of Camelot, grieving for his sister and raging at Uther, the king is already a broken man. Still mourning Igraine, and with an infant son he can hardly bear to look at, Uther readily accepts Tristan's challenge to single combat. The people think that he is eager to prove his honour; Gaius believes that he is eager to die.
Gaius watches the fight from his customary place – not from the seats with the rest of the people, but standing by the castle wall, ready to rush onto the field and tend the wounded. But his assistance isn't needed today; the fight quickly turns in Tristan's favour and before long the king lay dying on the field, red blood spilling on his red tabard.
"Sire," Gaius says, kneeling beside him and trying to staunch the flow of blood, but there isn't much he can do for Uther now. Nearby Tristan stabs his sword into the ground and stares up at a crowd that stares back, stunned and unmoving.
Uther looks at him as if he wants to speak and Gaius leans closer. "You must take Arthur away," he hisses through the pain. "Keep him from Tristan."
"I will do what I can, sire," Gaius murmurs, but before he can pull away Uther grabs his arm with one final show of strength.
"Swear it to me, Gaius," he says. "Swear to me that you'll protect my son."
With Uther gone, the child will be vulnerable. Although Gaius doesn't want to believe that Tristan would harm his nephew, he has it in his power now to make Arthur's life very unhappy. And Tristan is not the king's only enemy; Nimueh swore revenge when she was cast from the court months before, and there are countless others now since Uther, in his rage and grief, began purging the kingdom of magic. Gaius usually prefers to turn a blind eye to such matters but he's never been able to disobey his king.
"Yes, sire," he says, bowing his head. "I will."
Uther nods, his eyes closing at last, and Gaius rises and motions to the guards standing nearby to bear the king into the castle. Tristan watches them closely but doesn't move to halt the small, solemn procession.
Things happen quickly after that. The castle is as silent as a tomb, as if its very walls are already mourning the king, and it just takes a few whispered words, well-placed in loyal ears, before Gaius finds himself creeping through the tunnel that leads through the burial vaults, a bag of supplies on his back and the infant prince in his arms.
The sun is already setting before he manages to slip from the castle, and he immediately makes for the woods, eager to be under the cover of the trees. They don't make it far that evening; Gaius is not as young as he used to be, and there is the child to think about. But they're well into the forest before the sky grows dark, and in the light of the small fire Gaius looks at his charge, peering at the ruddy cheeks, the wispy golden hair, the sleepy blue eyes. Arthur has been mostly quiet since they began their journey, but now he looks fretful, as if he's just realised that he's left home for good.
"Poor child," Gaius says, rubbing his back soothingly until Arthur's eyes slowly close, his small dozing body a warm weight against Gaius' chest.
Perhaps it is his destiny to help this orphaned boy, to protect the rightful heir to Camelot's throne. Perhaps it is his penance.
Gaius has never been impulsive and has no real plan other than a desire to keep Arthur safe. He considers going to Uther's old friend Gorlois but immediately dismisses the idea, knowing that it's probably the first place Tristan, or anyone, would look for Arthur. Ealdor is perhaps the wiser option; Hunith is expecting a child of her own, her first, and Gaius knows he and Arthur would both be welcome.
But Gaius doesn't make a decision that night, and just a few short hours later he and Arthur are already moving on, the sky still dark overhead. Dawn breaks just as they reach the top of a hill, and when Gaius glances back they're still close enough to Camelot to see that the castle's banners have changed from Pendragon red to Dubois black.
He's never been Gaius the hero. He's never done anything of importance. But he thinks he can do something now. He can keep the child – the prince – safe until the right time comes. Until Arthur can meet his destiny.
With one last look Gaius turns away, fixing his eyes on the road ahead and trusting his feet to find the path to safety, as Arthur clutches his shoulders and looks back on the way they came, blinking in the sunlight at the castle in the distance.