The True Dark Lord | Harry Potter X LOTR

Eleven years ago, a cult of unknown origin and alignment birthed a baby boy shrouded in prophecy and power.

Mairon.

Due to the immense secrecy of this cult, Mairon's true parents were not known, not even within the ranks of this cult.

As such, no one could forsee what had just been unleashed upon the world.

In order to ensure the desired fate comes to pass, the cult left him, wrapped in a blanket, outside a muggle's door, and awaited the day he would leave an indelible mark on the world.

Throughout his early life, Mairon became aware of his gifts. Just a little.

His intelligence was rivalled only by his curiosity, and he didn't care about who got hurt when satisfying this curiosity.

(I'm not a die-hard LOTR fan or HP fan. If I get the lore wrong, don't get mad. Some things about both worlds will just have to change in order to accommodate Sauron in HP.)