I see, thank you for explaining your view, that makes things a bit easier. So for you a demon is a chaotic entity searching for a vessel to manifest this chaos in some way, am I correct?
Well, regarding the matter of immortality, there is a whole subforum on that matter:
http://www.occultforum.org/forum/viewforum.php?f=35
If I recall correctly the matter of merging with something was already discussed there somewhere so you might wanna take a look.
Now to come to the point of inviting something:
The thing is that you will have to bear whatever you call there. You are searching for something abstract and chaotic, possibly also as destructive as it is chaotic.
So you might take a guess of what will happen to you if you bear something like this and are completly unprepared.
What are the consequences? There are no studies or statistics on the matter as far as I recall, so a general assumtion is not really possible. A generalized approach based on some anecdotes aren't worth the breath. What I can tell you though is this: What you are dealing with in magick is first and foremost your own self. This to manage is already a big piece of work. It is not uncommon for a practioner to experience strong and extensive emotional backlashes and going up and down the psychic rollercoaster.
I would dare to say that it is not everybodies road to travel. Sure, some basic practices can be done by everyone, but deeper work takes a certain strength which is not always given. And what happens when you want to handle more then you posses the strength to so you will probably know from a gym when handling weights which are too much for you: You break.
I could serve now with a few anecdotes of people who lost their mind, developed many strange ticks and habits or started to develop paranoia. But I guess you already got the point. Yes, this and many more things can happen. It does not mean that it will but it can. As I said, in the practice one can experience deep going extremes which can be too much to handle.
Going back to the start: I already brought the weigth analogy from the gym. Nobody starts with the heavy stuff. You will get there in time if you keep training. You may also remain on a level you feel comfortable with or reach for bigger stuff. But this takes strength. And time. Don't rush in but take your time to find your position and the options you can reach from there.
Ramscha