The Erratic Sentries are found in the same place as the mobs for The Sanctum Wards: an arc starting at the Staging Area and proceeding broadly southwards along the western shoreline. They're non-agressive (yellow) mechanicals (pictured), and although they're level 70, it's still uncommonly easy to kill them. Sometimes they can be found reduced in health bandaging themselves up, and in general they move a bit oddly -- I guess that explains the "erratic" part.
After you kill and loot, target the corpse and right-click the Attuned Crystal Cores in your inventory; the Sentry will turn into a friendly one and stagger around briefly before disappearing. Note that you can't convert corpses that you haven't killed yourself.
Convert five corpses, and you're done. Return to the staging grounds for 9g, 150 Shattered Sun rep, and warm fuzzing feelings for helping get to Phase Two. This quest is still available in Phase Two, but renamed to "Further Conversions."