Evil spirits? I'm decently knowledgeable about the Goetic demons, which may interest you a bit. If you want to know a bit about some spirits, continue to read... it's quite interesting. I know this not to invoke any demons, by the way, but merely because it's an interesting topic and I enjoy writing fictional stories concerning the Goetia.

In all actuality, they may not actually be demons. Why is it that some people claim that they have seen demons and, if they end up failing, why do they continue to pursue it? You know ceremonial magic, correct, ADLIB? At least like, using the robes and mitres and burning perfumes among other things...well, according to the speculations of Aleister Crowley, the demons of the Goetia such as Bael, Asmodeus, and all of them, are portions of the human brain.

He says the seals are methods of stimulation or regulation seen through the eye; invoking God by his names are vibrations that are stated to establish general control of the brain, control of the brain in detai, and control of one special portion. The perfumes aid this through smell. The demons that are summoned are used for a generic purpose that is commonly attributed to them: attaining riches or healing a disease of yours. The only problem I have with Crowley's speculation is that, what happens if a 'demon' is summoned to heal another person's disease? Then, I suppose, it does not work.

What he says is, riches could be obtained through the brain enhancing business faculties, and healing diseases can be done through the brain regulating the body...so, basically, the summoned demons help you control your mind. This does not account for anything else, though. Who knows?