AIEEE! I LOVE THE IDEA!!!!
O_O
Is it bad that I'm already thinking about who I'd like to make sick? *coughcough*
Personally, I like the idea of Connlaoth being behind it....but we could always make it be a small group of connlaothians...people with a really strong hatred to magic. They don't ahve to be involved with the kingdom at all besides living there, that way the castle could really say they had no idea, they didn't do it, and TADA! you have a big crisis because they're not liars! I know for one that I have a character who's been looking for some sort of plot *coughtobrecough* and would be more than willing to sacrafice his freedom to kill off magic users XDDD
For symptoms, I think they should vary from person to person, because not everyone has the same powers so it would make sense that each individual is effected differently! i.e. shapeshifters wouldn't be able to control when and what they shift into, and could even get stuck as a half goat half dragon with chicken wings. Mind controllers and the like could have their magic backfire against them and they'll randomly control people and have them start attacking him or something....
Just an idea ^_^