Killed the undead boss mob, still have the “explore under the broken grate” step.
I didn’t get the explore the tunnels further step like someone mentioned above.
The pathing here is awful and the npcs and mobs go through the sewer walls.
edit:
I managed to get the “Continue your exploration” step the 2nd time, I swam through the broken grate, killed the river drake, then swam back up to where Warmaster Forgal was stuck and that seemed to have triggered the next quest step.