I think "us" just refers to the imps and the hordes of the bloodwar under the nautiloid. They later get boarded by followers of Zariel.
In my absolute speculation threadhttps://forums.larian.com/ubbthreads.php?ubb=showflat&Number=700069&page=2
sharp commented that it could be an elder brain, namely
I agree that the goblins turning hostile does not appear to make sense.