"You are banned from death." Celebrimbor remembered how those words had slipped from his lips when he'd needed a young Ranger's help. Now he found himself in the exact same position as the Ranger had been, unable to pass on and unknowing of how he was supposed to do so. For years upon years he had struggled in vain to find a way out of the endless deathless existence he lived where his mind alternated between softer and sharper. If he knew where the last of his kin were he would seek them out and request their help to move on to death. Unable to do that though he contented himself with lingering until he could figure it all out. In the end he wandered until at last he found her, the woman carrying the last descendant of the Line of Finwe.
The hand of fate lay heavily upon both her and the child that was just beginning to form. Knowing that it would be hard on the child and that if things went even the slightest bit wrong all would plunge into darkness deeper and more enduring than any Sauron or his Lord Morgoth had attempted to create, if only due to the lack of those who would fight for the light, Celebrimbor acted. He reached out and meshed his being with the unborn child's thereby reincarnating himself as the child. As a last act before his awareness dimmed he wove three spells around both the woman and her child. One to ensure that neither would perish until he was old enough to regain his memories, one to store his memories away for when his mind was again able to withstand them, and one to give the child an elven inheritance so he could again utilize his full powers but this would only happen once the second one fulfilled its purpose.
Everything would have gone smoothly save for one thing Celebrimbor failed to take into account. He failed to account for the killing curse and how it warps elven spells. If he had then perhaps he would have modified an object of theirs so he would have them fulfill themselves properly. Instead it was only by chance that he awoke again.