"of comfort no man speak"
The synflesh of his right hand starts to tear in the third month of his self-imposed exile. It's created, literally, to last but just like real flesh it breaks down. It grows old and rough, and he doesn't try to stop it.
Rather, he tears at it. The rough-hewn stone of this island is rough and unforgiving amidst this brilliant, beautiful sea (so much water, and even all these years later Luke is still in awe of the fact that this much water even exists) and his constant scrabbling along the old-worn paths has started to break his right hand apart. When he wakes up one morning it's to a long split along his palm which bears the whirring circuitry beneath.
A bionic hand. One to replace the hand he had lost at Bespin all those years ago.
This isn't the same one, though; this one he had gotten following a skirmish with a Force whip, of all things, when he had not had a shoto to help keep the separate tendrils at bay.
He starts to pick at the edges of the tear, watching disinterestedly as it pulls away with little pain. There are nerves built into it, of course, but they've become numbed from time and lack of proper care (it may look like a real hand but it still requires the latter).
He can still feel twinges from phantom fingers. He can still feel the lingering agony of a severed limb.
By his seventh month alone the flesh is completely gone, leaving the machinery exposed. He tinkers with it occasionally; it's one habit, one hobby, that has never left him. He shares too much of his father's blood for that.
He can hear Leia's voice in the back of his mind. She's a warm, familiar presence he cherishes but never responds to, even when he hears her ask him (rhetorically) why he left. Why he abandoned the galaxy when Ben Solo became Kylo Ren. She blames him in some small way for Ben's fall to the dark side, he knows, but Luke also knows that she blames herself more.
She hears rumors sometimes, of people wondering why the famous Luke Skywalker left. Some say he ran away to protect himself, to preserve the legacy of the Jedi Order. Others say he's a coward. Leia doesn't hold to any such thoughts, but she does wonder.
He laughs about that bitterly to himself sometimes, because if his own sister can't understand, then no one will.
He looks down at that inhuman right hand, at the cold shining metal, and he studies it carefully.
He's furious. He's frightened and angry and oh-so-infuriated, and he knows that if he tries to be the galaxy's savior now he'll fall so far and quickly that he'll never realize he's hit the bottom. He felt the deaths of his students, he screamed and wept over their loss in the rain as the fires hissed and spat, and he still wishes to go after Kylo Ren and the First Order. The shadows whisper to him and ask him if vengeance is really a bad thing, and he trembles remembering the awesome power of the Sith.
Once you start down the dark path, forever will it dominate your destiny.
Luke had always thought that Yoda had only meant him when he said that. Now he's not so sure. He imagines that his wise Master had understood the ways of the dark better than Luke could.
Ben is of the Skywalker line.
And Skywalkers often fight with the dark side. Anakin Skywalker's legacy is always to be of a man led into the nightmare of Darth Vader, the terror of the Empire and the murderer of thousands. His path has forever colored the perception of his descendants.
The metal hand whirrs and clinks softly, and Luke uncovers it to remind himself of what consequences the dark side can, and will, have.
He's more machine now, twisted and evil.