Dark Link's backstory, cut for length.
Way back before the creation of the world, all that existed was "heaven" and "not heaven." The goddesses and their servants, allies, friends, and occasional overmillenium guest lived in "heaven." Everyone else: demons, neutral factions, mask-riding demi-chaos-gods, etc., lived somewhere different. For lack of a specific Hylian term, I'll refer to the highest servants of the goddesses as Angels. Now Farore held a set of competitions to determine her champion should the need for warfare among the heavens, or on the newly created Earth present itself. Two Angels triumphed over everyone else, their names are lost to time, but we know them as "Link" and "Dark Link." Link won through skill, being faster in a race, out-thinking a maze. Dark Link won with strength and passion, the desire to serve the goddesses burned within him. He won his contests with strength; a race is easily won when the competitors are dead and a maze is no problem if there are no walls. Dark Lin beat Link in the final competition, a one on one fight. However, Farore chose Link as her champion. Dark Link went ballistic. He attacked Link, without a hint mercy this time, cursed the goddesses, and so they cast him out. They imprisoned him in his Hylian body, immortal, never being able to return "home." They never spoke to him, never answered his prayers. Eventually a plan brewed in his mind that would show his power and force the goddesses to pay attention to him. He would seize control of the sacred realm and wield the power of the triforce for himself. His army of who would later become the Twili were only there for distraction, cannonfodder as it were. He didn't care. He failed. The goddesses themselves came down to stop him, and for a moment, he was happy again. In those few heartbeats as he looked at the faces of the goddesses, he was ready to give it all up and go back. But it was too late. The damage had been done, and he condemned an entire people to a lifetime of darkness. Due to his celestial nature, he couldn't be locked away, so he was once again cursed to solitude. Only this time he was forbidden from using his real name. He then decided that we would kill Farore's champion and show her that he was the better choice. They first met in the Water Temple, where Ganondorf imprisoned Dark Link under the guise of waiting for Link to show up and be killed. Dark Link was in that same room, one with not-walls, no windows, no sun, no moon, no food, no water, for seven years. By the time Link came, he didn’t care about killing him, he wanted out of that room. Despite being vastly more powerful, he gave up and let Link win just so he could leave the temple. This, however, meant that his corporal body was destroyed. It was a simple problem to counter, he simply took up residence in a back corner of Link’s mind. For hundreds of years as Link manipulated Time and Light, Dark Link was always in the back of his mind whispering, “You’ll never make it. You’ll die. They’ll die. You’ll fail and your friends will die.” It only stopped when Dark Link grew bored of it and “slept.” When Link destroyed the Phoenix, it triggered a release of energy powerful enough for Dark Link to take form again. His first act was to attack Link. One thing bothered him though, during his time in the back of Link’s mind, Dark Link grew to see why Farore chose Link instead of himself. He makes the ultimate sacrifice out of his love for the goddesses. He stops, looks at Link, and simply says, “It’s over. You and I have been at odds for thousands of years and it’s time it ended. My name is...” And then was he struck down by the goddesses, never to utter his name.