Tinnurion is no evil persona, no servant of Morgoth. Tinnurion is a wandering elf, forsaken by his own kin. He is secretive, but no more than the Silvan Elves, hidden away as they are in their woodland realm. He cares not for the wars against Sauron, not because he secretly admires Sauron, he despises him as much as any other, but because he is put off by war. He fought in the first battle of the Battles of Beleriand and since then he has never fought in any of the wars of his kin again. He saw the world change before his eyes and he deemed it better to live a life away from that all. In fact, one could say he knew in his heart all the hardship that his kin would have to suffer in the days to come, and perhaps this aching feeling is what made him leave. I do not wish to explore his reasons more than is necessary, for not all reasoning can be so easily explained.
Within Eöl's realm he turned secretive and mysterious like his master, and the days went by as he lingered in the dark confinement of Nan Elmoth, and he learned to live away from the light of the sun brought about by the Valar in response to the destruction of the two trees. He turned inward, loving only the darkness of the forest and the light of the stars and moon as they lit up the night sky, and so too did he admire his master whose smithing work was beyond anything he had seen. Peaceful his life was in the forest of starry twilight, and he was content with his service to Eöl, who he deemed the closest thing to a father he had left. Alas things were once again to change with the coming of Aredhel whom Eöl lured into his realm, sealing his fate. Many years later his master was gone and did not return, and the nights became restless and the forest less safe, for its lord was gone and would never return.
Tinnurion grieved in his own way, and though it was not tears he is said to have shed, it was the closest thing to it. He fell into despair, hoping that his master would return to their dimly lit halls. He still found the bravery within him to venture out and look for him and in his desperate search he was intercepted and brought to Angband to slave away in the mines. His days turned dark in ways unbefitting even to him, and through hard toil and the whips of Morgoth's servants the light of the Eldar left him. But before he could be utterly broken, he escaped. It is said he wept as he fled across the fields into Dorthonion, from whence came the Noldor and took him prisoner. And all the hope he had of escape shattered as he found his hands bound once more. For a moment he forgot all hatred towards the Noldor, kneeling before them, begging for his release. The things he had seen in Angband frightened him and he chose to share it with the Noldor for the betterment of all, but they would not heed his words of warning, treating him with disbelief and disdain. In response, Tinnurion portended that their arrogance and misguided behaviour would see to their ruin. As he was of elfkind, and the laws of the Eldar were clear, they could not slay him for such threats. Instead, they decided to let him go, though had scouts tail him for many leagues southward.
Henceforth he turned a wanderer who would never find a home, and the stars would be the only memory of days long gone. He left Nan Elmoth east to flee from the wars that raged in Beleriand and while he found friendship in the dwarves throughout the second age, it was lost to him when Durin's Doors were shut, and the few dwarves whose respect he earned after those days would perish one by one, as is the fate of their kind, and the days of their friendship would pass into memory. So it was that Tinnurion became more and more distant and cold like a winter's night with no stars, and whatever light he still bore of days of yore in the realm of Doriath was now completely fading and it alarmed him greatly. So too he grew ever more restless about the fate of Middle-Earth at large, even under the comfort of night, for he saw that the world was changing rapidly. He had seen it throughout all the ages but had never loved it more than when it was youngest. When Sauron was overthrown, and the time of the Elves came to a final close, Tinnurion started to realise that the only way forward was to go back. He did not wish to be left alone to see the world grow old in solitude and to wander the lands of Middle-Earth until he would fade away and his spirit would linger evermore in the forests until the end of all things.

