Sewer monsters?

[Rechan]

A group of kids looking for a ball that went down a drain.

Ghouls are great. And ghast smell might be mixed with something else.

I love oozes. They can get a smorgusboard, just consuming refuse. A gelatinous cube in a pit makes a great garbage dump. Even better, imagine some criminals disposing of a body.

Goblins picking through trash hoping to find something useful.

Aranea meeting.

Some subterrainan race (Grimlocks, Drow, etc) prepping for a surface raid.

Really angry water elementals.

 

[Dannyalcatraz]

A group of kids looking for a ball that went down a drain.

They all float down here!

 

[TheAuldGrump]

Possible hazards include firedamp (methane+torch=*FOOOMP!*) stinkdamp (if you smell rotten eggs in the sewer then leave the area - concentrations of sulfur dioxide are really bad stuff), sinkholes hidden in effluvium, loose stonework above, infections and diseases, and yes, giant, blind, albino sewer gators! Sewer octopi work well too.

Misty type undead, concealed withing the mist rising from the effluent on cold days (what Egon would call a 'free floating vapor'). Will o the Wisp work well, their glow diffused by the same mists that hide the ghosts.

Transients using a disused channel for shelter. And the seldom used Dark Mantle drifting down from above.

The Auld Grump, who also likes the image of a Grell squeezing out of a pipe that appears far too small for it to fit. Watch an octopus get in and out of a pop bottle to see what I mean.

 

[Orius]

I'd say everyone pretty much covered the bases. If you've got a sewer, you need the dire rats, wererats, and naturally the gators. And otyughs, oozes, and gibbering mouthers probably are pretty good too.

Unless of course you want to surprise the players with something unexpected.

 

[Tewligan]

C.H.U.D.s.

 

[Scarbonac]

This guy:


[Edit: I must be blind.]

 

[Whimsical]

Slimes, Molds, and Fungi.

 

[Cyberhawk]

If you want some encounters with a bit more flavor try these:

Not only could you find criminals down there but the sewers would be a perfect spot for a hidden shrine or temple to a forbidden god. Especially the disgusting ones.
Maybe the PCs somehow stumble in just as they're about to sacrifice some poor sap.

Sewers are a traditional spot for dumping bodies. Dumped bodies=undead. This could create a nice spot for a ghost with a backstory or adventure hooks of some kind.

Blind Albino Owlbears. Kids buy the chicks at fairs, you see, and then they dump them in the sewer when they get bigger and nastier. They adapt down there and they get hungry. A friend of a friend of my adventurer totally ran into them once...

 

[Neil Bishop]

If you have a Libris Mortis, a sewer could be an interesting place for undead with the swarmshifter template. I can just see several swarms of undead flies or rats transforming into a ghoul pack.

Full disclosure: one of the leading crimelords in a city in one of my games is a swarmshifter human wererat vampire who can assume the following swarm forms: bat, rat and fly as well as a living spell: contagion.

 

[Rechan]

The Auld Grump, who also likes the image of a Grell squeezing out of a pipe that appears far too small for it to fit. Watch an octopus get in and out of a pop bottle to see what I mean.

The only problem there is that the image will likely be lost or unappreciated; PCs would more than likely just blast it as it's crawling out of the pipe than watch.

(Unless the thing can move real fast).