Found near the entrance to the zone on the left tower. Easy to solo camp for a 60 Necro mostly Kunark geared: shove yourself and pet in to the corner closest to the wall, sic your pet, and begin throwing DoTs. Touch of Night to keep yourself up.
Spawn timer is approximately 19:45 to 20:00 today on Mangler, so about 3 spawns per hour. As stated by other posters, you'll see one of three mobs: a Di'zok Sergeant, Captain Di'ouz, and Watch Sergeant Riz'oul. During my camp, I went 15 spawns without seeing a single Captain Di'ouz, so I'd put him on the rare side for spawning--- maybe 10% chance or so, but my sample size is quite small.