Warrior Poet: Yeah, I'm using the Blackfire. I've given it a Will Save (DC 22) to prevent it from attaching to you, with 1d2 permanent con damage each round. It still requires three succesive saves to put out, or being subjected to at least 8 dice of damage from an instantaneous elemental source (i.e. lightning bolt, fireball, etc.).
There's only one fighter type in the grou now, but he's a paladin, so yeah, the will saves are heavy on everyone. That's ok though, noone in the group is immune to the blackfire, and it will still be hard to put out if it hits them. Not to mention that even a couple of points of con loss can hurt pretty bad.
I don't think I'd like to give the necromancers enough understanding of blackfire to use it as a weapon just yet. However, if they're still around in a while, and ever manage to get 2 weeks free of assaults by the party to study it, they'll have perfected the art of protecting themselves from it. Then the party may have to face fighter-types who are wreathed in blackflame and can transmit it via touch. That would be especially mean on the troll fighter in full plate that gaurds the front door.
As for Blade Barrier being an illusion, my party is more likely to try to dispel it than teleport past it, as the only tracvel magic they have is teleport, and a 5th level slot is hard to come by. but still, they tend to group together when passing through doors, so a blade barrier will get them all in its area. The rogue that is now a vampire, as well as the leader of the necromancers both know that from previous experience with the party, so I don't feel guilty using DM knowledge against them.
Dougal Dekree: The party has tons of weaknesses. Primarily, they're overconfident. If things appear to be going ogood, they'll press the attack.
Also, they don't like to leave fallen comrades behind.
Finally, most of their strength comes from magic.
Mmmm, greased quicksand floors. . . The party even created a hole in the floor the last time they were there. Well, they emptied one that was already present at least. And it should be fairly easy to find quicksand in a swamp.
Old One: The only cleric on the bad guys' side is out of action for a while. However, as soon as he's back, silence zones will become a serious threat to the party.
The necromancers don't have fast access to ropers to kill. However, given enough time by the party. . .
I've used (3) against the party before (not the rot grubs, but the slime). Where can I find a conversion of rot grubs?
As for (4), nearly ever major enemy can teleport or otherwise retreat quickly, so there isn't much reason for them to destroy their own home to stop the party when they can always retreat and come back.

I think I will have one area where a guy is ready to do that though, perhaps the approach towards the Headmistresses rooms?
(5): Definitely! Heck, the area is fairly low on fighter types, so the ones that are there have major buffs on them too (magic vestments, greater magic weapons, etc.) Every wizard there knows at least shield, and if they're high enough level, they also know shadow Images or Mirror Images. Shadow images is from R&R, and is like Mirror Images, but with fewer copies. Each copy has 20% of the caster's hit points though.
