He was born to the royal Sunstrider dynasty hundred and twenty years before the orcish invasion of Azeroth. His older brother, Kael'thas already was already an adult and took a liking to his brother. All was well durring his childhood, his mentors trained him in the ways of magic, and he learned of many of the high elves' ancient stories and their mysterious heritage. He was taught in the ways of the Sunstriders by his father, and later by his brother, who combined the slinging of powerful spells and excellent swordsmanship in combat.
Durring the Orcish invasions of Quel'thalas Eldin fought valiantly, despite the effort of the Amani trolls and their orcish allies. He faced many decoys durring this war, some didn't end out as lucky as others. The Trolls overpowered his troops near the end of the war, but he was saved by the Alliance forces the same day Zul'jin was captured. He withdrew from the war himself, returning to Silvermoon to heal.
For the next fifteen years (After he had been fully restored from his attack and imprisonment), he would hunt down the trolls, as an act of vengeance. He lived with a pack of five Farstriders for years. Though, when the Undead Scourge assualted the borders of the elven forests he had sworn to protect he changed his focus. He wanted to exterminate this threat swiftly, taking commands from Sylvanas Windrunner. His group could not resist the undead for long, and he was asked to run to Silvermoon by the three remaining rangers, as they would watch his back.
The rangers didn't hold much of the Scourge back, and Eldin moved to the west instead of the north, to stay safe from most of the undead. He hid in a desolate cave in the woods, he wasn't found by the undead forces. He thought that Silvermoon was destroyed completely and that it's people had died, or worse. He fed himself on the odd plants, growing outside of the cave, of most he didn't know the name, but he could recognize most as non-poisonous. After days he was hungry, despite the fact that he could feast on all the fruits and vegetables he wanted. This hunger came not from the stomach, but from the mind. This was a hunger for vengeance and justice. His eyes turned pale-white.
He travelled back to Silvermoon, finding it odd that he only had to slaughter a handful of undead. Near the gates of Silvermoon he found a Farstrider camp, which protected his brother and several other important figures of Silvermoon who had survived. He himself turned to fire magic instead of arcane, frost and water magic in order to fight the undead better, as he was instructed to do by his brother. He frowned upon fire magic, as it resembled the demons of the ancient stories of his father so much. His magic also included fel magic as of then.
He was tasked to turn to the Kirin Tor weeks after Dalaran was destroyed, he sought for the towns miles away from it, escorted by naga and other blood elves. The escorts were killed violently, and Eldin was taken to Ambermill to be imprisoned for the usage of Fel magic, and affiliating with the serpentine demons, the Naga.