Don't hate me! I'm going by practicality and Merlin IS Immortal.
The path was eerily dark. The overhanging trees diminished any light given by either the moon or stars. To anyone who wasn't accustomed to such a setting would find it awfully difficult to move about. But as it is, Merlin has walked this path plenty of times before. The last time he had was to say a final farewell to his longtime companion and friend.
Within a clearing, three kilometers from the entrance to the forest, there is a large stone. On this stone remain some of the hardest words Merlin has ever had to write.
Here lies Aithusa
Faithful friend and longtime companion
Last of his kind
He will be remembered
1287 years
Last time Merlin attended school here was not to check on it. He wished to spend as much time with Aithusa before he left this world. The one constant Merlin always held dearly was finally gone. In 1811, it was Merlin's final year during that stay at Hogwarts. At the time he had gone by Marcus Pendren. School passed without any effort. He has been taught the material time and time again. His main focus was Aithusa.
Backs then, the restrictions placed on the 'Forbidden forest' were not as tightly held. There had been no Tom Riddle to strike fear into peoples' hearts. There had been no 'Death eaters' to endanger the students. To venture into the forest was to risk one's own apparel. He would visit Aithusa's clearing every night. The two would share stories of their time apart. Merlin about all of his travels and Aithusa about all of his surprising experiences with adventuring, unsuspecting students who had wondered upon him after distancing themselves too far from the school. After everything, the two old friends had a good hearty laugh on the matter. But his last visit was met with tears. Merlin had not been the only presence with the 'Great' Dragon's passing. Plenty of magical creatures from the surrounding forest that had also grown accustomed to the dragon's presence among them had come to pay respects. All gathered around as Merlin carved the words into Aithusa's gravestone and stood back in a quiet silence.
Merlin noticed that the mood of the forest has not been the same since Aithusa lingered here. It seemed darker and sadder. The creatures that used to converse openly now kept to themselves. Many of those alive today are too young to have met Aithusa but those who remember him still acknowledge him in all his glory.
Merlin doesn't know how long he remained there just sitting at the stone. Not until he looks up at the moon's placement in the sky does he realize it's been hours. With a deep sigh, he stands and begins the trek back to the castle. The lingering silence has been a welcomed comfort. After a couple months of rooming with several rambunctious teenagers, this is a blessing.
He just reaches the portrait of Camelot when he sees a figure approaching from the other direction. As soon as it gets within closer proximity, Merlin recognizes the figure as Arthur. He appears to be carrying a vile of liquid in his hand. The two awkwardly gaze at each other for seemingly outstretched seconds before Merlin utters the password 'Camelot' and they both head up the stairs. You could literally feel the tension between the two. Merlin's uneasiness of beginning things all over again and Arthur's uneasiness after what occurred during dinner just several hours previously. As a means of truce, both seem to nod to the other and head up the left set of stairs across from the entrance to their individual dormitories to settle in for the night. All either of them can think is, this is going to be a very long year.
I always like reading about stories with Aithusa in them but for this I wanted to keep the facts as significant as possible. In the show, they established that a dragon could live to be around a thousand years old. Aithusa outlived that but he could not live forever. Merlin is the only real immortal of the story.