From the very beginning, it was pretty clear who the final boss was going to be in Salvation's Edge, the newest raid in Destiny 2. There was no way that players were going to be able to defeat the Witness on their own in the campaign, as it is far too powerful. So, before the combined forces of Sol can destroy the Witness, they must first break its hold on the Traveler. That is exactly what the final encounter of Salvation's Edge entails.