Hunter of Sewers of Nihilia (10 Points)  

In Barindu near the southeast corner of the yard, hail Gamesh and choose your sewer expedition: "the treatment [plant], [crematory], [pools] of sludge, or the [lair] of trapped ones."

In all 4 sewer instance zones, you need to kill 20 trash mobs to start with to cause the rares and/or placeholders to spawn. Rares have multiple possible spawn locations throughout each zone. The sewer zones are spread out enough that even the best trackers have to move toward the center of the map to track the whole zone at once. Once they've popped and you've killed any rares, you can choose to stay in the zone killing placeholders, or you can quit the expedition and start a new one. I chose the latter. To exit the sewers, click the ladder near the zone-in, or quit the expedition with /dzq

Plant:
The first placeholder in the Plant sewer zone is a turepta in the third room, just before Ansharu. Once that spawns, you'll know all the placeholders have been triggered to pop.
Grimy Turepta
Kril V`lak
Ukun Packleader

Crematory:
The first placeholder in the Crematory sewer zone is a nihil spirit between the first two bends of the first tunnel.
Cryxtetl
Diseased Larva
Slime Devourer

Pool:
The first placeholder in the Pool sewer zone is a critter (gnat, larva, stonemite) in the middle of the the south ledge in the second room.
Distraught Slimesnipper
Festering Sewer Gnat
Xthid the Undying

Lair:
The first placeholder in the Lair sewer zone is a nihil spirit in the middle of the first tunnel.
Blistering Larva
Rigara the Lost
Sludge-Eating Fly