The pre-event with escorting the Dervs is bugged, they are standing at the Staging Platform and not de-spawning when it's time for the Death-Branded Shatterer, so no event starts.
Several collections are dependent on killing the Death-Branded Shatterer, so no progress on those are possible.
This is happening in other instances of the map as well, actually on most, the Dervs are not there at all.
From looking in this forum I can see it has been an issue last year as well.
Hope it gets fixed soon