I can see it like this.... In Destiny 4: The darkness has started to lay siege on the earth, right after we return from a suicide mission from Venus (Venus at this point was completely engulfed in the Darkness, making every move a death trap. Potential timed escape missions were present) with the Heart of Atheon. The Traveller (awoke a couple games/expansions ago) is working with the Warminds and the Fallen to keep the Darkness at bay, but the Fallen are being torn apart, even with their new strength from the light, and the last Warminds (again, Rasputin. This bastard is always the last to die) is on the verge of being completely destroyed. The suicide mission you narrowly died on was all for the last ditch effort to stop the darkness. Knowing that the assault from the Darkness will destroy everything, the Traveller uses the last of its powers and infuses the Heart with all its light, allowing it to send exactly one person back in time to before the Darkness became the cosmic entity it is today. This will kill the Traveller and Doom the planet earth, but if you are successful in stopping the darkness, then it will all be worth it. They also warn that sending you that far back will separate you from the Light the Traveller supplies, meaning your Ghost will cease to function after a few minutes and you will eventually die. The Traveller swells up all the light it can muster and feeds it into the Heart. Once the Heart takes the last of the Travellers light, it tells you to quickly touch the Heart and wishes you luck before plummeting into the earth, sending a massive shockwave towards the Tower. You quickly touch the Heart, just as the shockwave obliterates the Tower. Everything flashes by you. You see many things that have been and have yet to be. Then, everything bursts into a flash of light and the images stop. You open your eyes and see.....darkness. I'll leave the rest for your imagination lol

We go back in time to do it all over again woot !!

Not quite! This is back in time before our ghosts find us. Before the the darkness takes over. We put a stop to it before it even begins because we realize we are no match for it in the future, it has become just too powerful. Our only option is to travel back in time and fight the darkness before it starts to spread like wildfire. We defeat the darkness and change history, the ultimate sacrifice for mankind. Since that present time has a new time continuum we never really existed.