The End (S06E17)

For various motives, both Jack and the Man in Black enlist Desmond to snuff out the light at the island's core. As the island begins to crumble, the Man in Black becomes mortal. In his desperation to flee, he engages Jack in a fierce battle, severely injuring him. However, with Kate's assistance, Jack manages to defeat him. Coming to terms with his fate, Jack sacrifices himself to reignite the island's heart, enabling his friends to escape on the Ajira plane. Hurley steps up as the new protector of the island, appointing Ben as his advisor. The flash sideways is unveiled as a form of limbo, where the characters linger until they reconcile their lives and deaths, allowing them to "let go" and "move on" together.