Zycopath

Or everlasting life, so Caligo frees you from this plane.(Imprisonment)

This is very similar to one of my ideas regarding the same wish. The only difference is that I planned it as a modified Time Stop. What really happens is that Caligos puts the PC to sleep and traps him in a single moment (he speeds up CHARNAME's perception of time so a moment of real time seem like an eternity).
Another idea of mine is that CHARNAME wishes to return to Candlekeep and is sent to an illusionary Candlekeep created from his own memories resulting in that everybody acts just like they did before CHARNAME left at the start of BG1. This would probably be the most fun for players who played BG1 recently and can recognize the innkeeper's (Winsomething?) jokes and the dwarf's ratslaying quest. Besides reliving old memories this would likely also involve a quest of some sort to return to the real world.

Oh, and another thing. Since this is my first serious mod and it seems to be getting waaay bigger then I first pictured it I'm wondering if anyone's interested in helping out. If not, I'll carry on by myself and it will eventually get done but some co-operation would probably speed things up by a thousand years or so.

What do you think of an illusionary weapon that doesn't do much damage but is really fast? If nothing else it would probably be a good mage-disruptor.

And lets not get stuck on weapons, I'm still very much looking for other wish-suggestions and the price to pay for the wishes.

But an illusionary sword wouldn't really turn to dust, would it? It would just fade away or something. This easiest way would probably be to display a message on the screen, something like "You see your fancy new Hack'n'Slasher + 13 fade away into thin air before your eyes".

Another thing regarding the rewards, it would be kinda evil of me (and Caligos) if all the rewards turned out to be bad in some way (the player would get a bunch of XP though). Anyone got any ideas on illusionary rewards that could be good and permanent but still a twist on what the PC wished for?

You could implement a magical souljar mechanism.  You code it so that the tiefling gives the player a souljar that will "capture" any quality souls he kills.  Then you apply a global to the death of various game npc's.  In dialogue with the tielfing if a certain global has been set you can choose a reply related to that soul..e.g you kill Garren's child and set global garrenkidsoul 1, then in dialogue if garrenkidsoul 1 the tiefling would say "Ah, the soul of an innocent child, how delicious!".

Great minds certainly think alike. This is more or less exactly how I was planning on coding it. I could have sworn I wrote that somewhere...

As for where he'll be placed it's probably going to be somewhere in the city and perhaps somewhere in ToB at a later stage.
I have considered weapons with some sort of drawback as a reward but just giving it low damage seems a bit boring. More ideas regarding this would be nice.

I'm in the middle of writing Caligos' dialogue and I could use some help. Since a lot of the dialogue is going to center around his request for souls and his granting of wishes I could use some suggestions regarding that. First of all what CHARNAME could wish for and how Caligos could grant it (remember that everything he does is illusions, although really powerful ones) and also what souls he would ask for in return. Some of the souls would probably be from existing NPCs and some from new ones, ideas are appreciated here as well.