I downloaded the Vile Lair addon. In addition to it being the most boring addon to date, there is also a problem with it. I don't know if it will affect everyone but I thought I would report what I found.
When you furnish the place, you will get some plants to harvest from like the ones in Frostcrag Spire (another addon). One of them is a Chokeberry. The chokeberry has one effect called Poison Fruit. It is used to poison others when they eat it. Just testing its efficacy, I ate it and it damaged 10 health every second for a very long time. I healed and took restore health potions until the effect timer ran down but to my dismay the effect didn't go away! I visited a chapel and took cure poison potions with no effect.
So ... be warned ... save before doing this or you may be sorry. Or just take my word for it and don't eat it at all. Luckily for me, I save quite often so only lost a few minutes of game play.