One day, on Notch house, Notch and Herobrine go to the pool for swin.
And they make a game, Who could stay longer under the water wins. Herobrine won the first after the second round Herobrine disappeared actually drowned. And got white eyes. Time out of the pool and immediately called his father and they saw that he was gone.

Herobrine is immortal, but he's just dead by the legendary sword Notch created by his brother Notch