power of the soul, spirit fushion Options

Iv been wondering and this is just a strectch if there is such an art of mystic ability that allows a normal human to bond his spirit with that of the fiends around him the devils that are in physical form would it be possiable for a person to be able to hold the spirit within themselves and then use it when ever they want with a degree of negetive effect but still im just wondering iv seen it done is some bits of fiction and also have read in some obscure mystice works that it is possiable i would just like to hear from others and if so how to do it would not mind learning myself

I'll try not to let my personal feelings cloud my answer....but sometimes you guys make it tough!
What you are describing is a form of 'controlled' possession. There are ways of tapping into abilities without the use of demonic entities. Spare used 'Atavistic Resurgence' to do a similar act. The spiritual dangers notwithstanding, there might be a distinct possibility of either creating or encouraging a schizophrenic split in ones psyche. I would caution that one gets or experiences some fairly intense psychotherapy or contemplative meditations before embarking on such a quest.
I could theorize 'til the cows come home...I guess that my initial question to you is Why? (IMG:style_emoticons/default/shok.gif)

Offtopic side note: Happy Belated B-day! I hope it was great!