This is a fictional item, and is not in Minecraft.
The Sword Of Ender is a sword that was forged by Endermen, and given to Herobrine Reborn. Its first victim was Steve, and it made Harry Reborn out of Steve's Wither Dust. It only appears in the Scar Realm Trilogy, and not in the actual game.
The Sword Of Ender acts like a normal Minecraft Sword, in that you have to stab somebody with it for it to deal damage, but then makes its victims eyes turn white like Herobrine's, and they explode into wither dust which is then sucked up by the sword, giving it power. Its holder has to have the Breath Of Ender, something which will allow you to become immortal and powerful.