I'm a brand-new DM (long-time player) writing a one-night 3.5 module for 11 people leveled 5 to 6. I could really use some help on building this villain, and in such a way that will accommodate and challenge a large-party arrangement.
What I need:
1. Suggestions on cool abilities for Teorna that make sense for her past
2. Fill in the blanks below!
3. What CR to give a villain that's facing 10 or 11 people, since CRs don't scale to larger parties.
Perhaps players find items that remove some of her powers which effectively lowers her CR. Or perhaps she's a CR 8, with two CR 5 animal companions? If the animal companions is the way to go, I'd want them to be able to bestow some sort of disease.
Teorna, the Bramble Queen (CR ___ )
Medium-Size Elf (Evil, Lawful)
Description: Teorna was once the beautiful daughter of the Druid-Lord Faroth, Guardian of the Sacred Forest (a forest which contains good and evil artifacts). After her father was banished to the deserts for trying to take the power in the forest, and her secret relationship with a nearby prince discovered and forbidden, Teorna's grief consumed her. She began to destroy the life around her; flowers wilted at her very presence and animals grew sickly. Growing ever more dangerous, and more twisted, the druids imprisoned her within a circle of massive thorny trees not to be freed for hundreds of years. Her seething hatred grew and after pleading with the evil gods for vengeance, she grew into a powerful creature of evil and escaped.
There's a really interesting prestige class in Complete Divine you might want to check out called the Blighter. The idea behind it is a druid who has lost her reverence for nature and has instead developed a destructive relationship with nature rather than a fostering one.
I've never played one, since I don't usually play evil or druids, so an evil ex-druid isn't really my style, but it might be just what you're looking for.
Thanks Eagor 
That prestige class is pretty close to the concept I'm going for.
Still hoping for some help filling in the hard numbers. I've given it some thought and I'm thinking of making it a CR 9 or 10 with around 200 HPs and an epic attack that could kill a Wizard outright, or take a fighter down to half.
Any other ideas?