Hey I have an idea for something that would freak some people out.

In the Canal area you have all those shadow mushrooms. Why not turn a few into Shriekers? Remember them? From AD&D 1st Edition? They used to drive me nuts in dungeons. I think it would be fitting to have some of the shadows do that. Here is a link to the 3.5 stats on them for you. http://www.dandwiki.com/wiki/SRD:Shrieker

Also you might want to look at the Shadow Mastiff as a monster to run into on some the Quests in the Canal or other areas tied to Shadow. (page 162 MM 3rd Edition)

Just thought I would throw my 2 cents in.

There are already Shrieker's out in the Underdark. It doesn't make much sense for the Shadow Mushrooms to be Shrieker's as they are in the middle of the city. They'd be Shrieking constantly.

Depends on where you put them. If they were guarding an entrance to a place you did not want people in, then they would make sense. That could be the work of the kolbolds in the Canal that have a Shadow Dragon statue in one building.

Plus this is the Underdark, and they are an underground monster that works with purple fungus so both can survive.

Add to that the entertainment side of it, like the surprise when someone goes walking around the corner and that Shrieker goes off! I don't know about you.. but I play via headphone. I know it would make me jump sky high!

There are, as pointed out, already Shrieker Mushrooms at various places in the underdark. Most players have played enough to simply not care anymore when these things start shrieking, and putting them in the city wouldn't really make much IG sense, since the Mushrooms in the canal were originally grown to shed light. (( Ie a different species of mushroom than shriekers))

Shierker mushroom in the city would be cut down pretty quickly.

Letsplayforfun Shierker mushroom in the city would be cut down pretty quickly.

*Angry commonor's swarm across the hatred mushroom with halberd's and bastard sword and destroys them*

Indeed, those mushrooms were planted as a light source and were definately not shriekers. However, if you want to try you could always try cultivating a shrieker or two to guard something. There are certainly enough knowledgeable druids around to help.