As the PCs explore under Vigil, I have divided up the sewers into an upper, relatively safe level and a lower, mostly forgotten level. The lower level is where the Seal Breakers dwell.
Along the way, I decided to place an additional encounter to give the sewers more flavor and the party more thematic hints. I set up a nest of otyughs attended by akaname. The otyughs gather because one of them is a plague-bringer otyugh who is afflicted by visions. She is also a devotee of pharasma (quite unusual for an otyugh) though she refers to her as the "lady of corpses". the akaname is also led by a priestess, who also follows Pharasma. The plaguebringer otyugh and the akaname have a symbiotic relationship. The otyughs provide protection for the akaname and the akaname interprets the otyugh visions and provides comfort to the plaguebringer (who is in constant pain).
In our party we had a cleric of Pharasma and she hit it off with the otyughs (about as well as you can expect). I had the plaguebringer recite a prophecy to the party, which went as follows:
A great beast grows under the city
A heart of nothingness; eyes of undeath
The two become one before the cleansing
The eyes rend from the heart, each stab at the other
The sewers destroyed, fire from above
One is enough. A dagger through her heart
The bloodless' revenge upon the bloodless.
Its partly generic, partly tailored to my party.
As an added wrinkle, the area is along the seal-breakers' patrol route, so the otyughs have no love for them. I ended the encounter with an attack on the otyugh complex by one of the seal-breaker patrols.