The 7th Gem of Harmony, the Gem that has been sacrificing everything to protect the land of Equestria was never been saved of its last remaining history or stories, for his destiny was very irrational, no pony knew what has to become of his great destiny, not only that but his greatness upon the tree of harmony, for you see the tree actually has 5 branches, 1 core and 1 large special root that connects to all the 7 Gems, the root of the tree is the 7th Gem, the Gem of Sacrifice.

This Gem was the one that has been sacrificing not only his life but his very soul as well, to rid the evil upon which had lurked here in the land of Equestria for years, actually thanks to him the tree of harmony was created before the birth of Celestia herself, but after that he died upon the fatal wounds and incisions that weakened him on battle against the evil, lurking around the shadows of the very birth place of Canterlot, no not birth place but time.

He died protecting his friends, he knew that they are needed to be alive at the war of the Light against Darkness, he knew that they are needed to care a very special filly and the savior of Equestria, The Day Bringer of Holy and Light…

But before his death, he wrote down a very special spell that allows him to connect a very special pony on another dimension to this very world, forcing him to go to this very special place he knew he would find greatly to live in, he already casted the spell upon reaching his death on the war, in that spell causes a massive light that brought the peace to this world, he knew that he needs to die for this spell to work because he foresees that no pony shall ever remember him at the day of his death, for his spell takes the happy memories and thoughts of him being with them in their past lives.

He could have just find another way to defeat the evil, but this is the only way he could ever think of,
taking his life and memories in all of his friends and families, gaining the right amount of energy to end the war, and win for there very great land, causes him to be the mystery and the missing Gem of the Tree of Harmony.