This is a fairly short mission, but one that is quite important in terms of plot, if not gameplay.
Following the prior boss battle against the Great Shard, you will be visited by Theresa and after some heated conversation Reaver is finally encouraged to join you. Once the scene ends, you will all leave together and be transported to the hill atop the Heroes Guild.
After one more conversation, walk forward into the centre of the hilltop and stand upon the circular platform next to Theresa. Watch the ensuing scenes.