The fact that this dream gets more detailed and longer everytime you have it, means it has an important message for you, and because you don’t understand the message yet, the dream keeps coming back.
I’m not pretty good at explaining dreams, but I’ll give it a try. The important elements here seem to be:
- The medieval dungeon/prison
- The small beat up mirror
- You having long silver hair and glowing green eyes and being quite pale
- A man with long brownish hair, who’s at the same age and who’s sitting on a cot
- His eyes are glowing blue
- Upon locking eyes, you’re overwhelmed by feelings, mainly bitterness and kind of hatred but also some sort of connection
- In the end, he half smiles and shrugs
I cannot explain the dream for you, because you’re the only one who can accurately read the symbolical language of your dreams. However, if this were my dream, I might explain it like this: first I’d ask myself if I know this man irl… he doesn’t necessarily have the same look irl, but perhaps the feelings towards the DC are the same towards a person irl. The dungeon can symbolize a hidden dungeon in the psyche, way below the surface in the dark corners of the mind. Because it seems like a prison, perhaps it means I’m trapped in there… the cell door however shows there’s a way out. The mirror is a symbol of selfimage. The mirror was beaten up, so perhaps I suffered/suffer from low self esteem… I also might think of myself as being a failure, who can’t do much good. The pale image also suggests that: I see myself as only a pale shadow of what I truly am. Now, the glowing green eyes… can you perhaps describe this aspect in more detail? How does the glowing feel? Is it like a bright light, or does it have more an aura of gloom? Whenever profound colors appear in dreams, I always see them in correspondence with the chakra colors. Green is the color of the heart chakra; because I’m stuck in the prison, the green glowing might suggest I have an unbalanced heart chakra, but I’m craving to restore it, to see it. Deficiency of heart chakra activity results in bitterness, shyness, isolation, loneliness and criticism. One of the major causes of such deficiency can be grief, and the heart chakra is fully developped between the age of 4 and 7. So perhaps I’m suffering from a childhood trauma which resulted in isolation, bitterness and shyness which then resulted in low self esteem. But I’m craving to break through this prison of the mind, and restore my relationships with people. Also, I might be craving for love, which would certainly break the cycle. However, then this man appears, sitting on the cot. The cot can be a symbol of early childhood events, which may have some connection with the man. He has the same age… perhaps he was a little friend during early childhood? Perhaps he was the cause of my isolation and low self esteem? His eyes are glowing blue, the color of the throat chakra, a symbol of communication, self expression. How would you describe the glowing in his eyes? Is it the same as in yours? Do you fear him? Perhaps this man tries to communicate again with me irl, but he keeps me from walking through the cell door. Although his intentions may be upright… I haven’t forgotten the bitterness and even hatred towards what he might have done to me when I was a little child. I’m overwhelmed by emotions when I see him, but I don’t really understand the connection I have with him. Perhaps I don’t recognize him irl for what he truly is? His smiling may be a sign of his upright intentions, but could also reveal more darker intentions… perhaps he even wants me to keep here in the dungeon??
If I could reenter the dream, I’d certainly try to reconcile with him and talk about the situation. Hopefully he reveals his intentions and helps to break down the prison.
That’s one explanation of the dream symbolism, but only as seen from my own perspective, and only if I had this dream. I translated it with my dream language, not with yours, because you’re the only one who’s capable in doing that. So there’s a good chance this sounds like complete nonsense 
But still, does it make some sense to you?
So strange, that the dream appeared before you played it… I wonder if the game already existed when you saw the dream the first time. Maybe you could see somewhere an advertisement or something like that… I don’t want to say that the entire dream is just a remembered ad, it must be more important… 
It’s just interesting. But I believe different people can have the same idea about at the same time, like they accidentally connected to a wrong postbox… 
Maybe someone people who created FF had once a similar dream, too… 