We have a few important things we need to ask before doing ANYTHING.
First:
They told us their name. They did not, in fact, tell us who they are. They know who we are - an elf maiden and fledgling necromancer. We do not know their identity. What ARE they? How are they here? And most specifically, why do they need us? Why couldn't this creature do this all himself? Fale Packunion... is that a dwarven name? (If we think it might be something else, say that instead, but it sounds dwarven to me)
Two:
Why do they need to know where we are? Even assuming we wish to work with them - something we do not yet know, since we have only the words of a dream avatar to go by - can we be sure the other necromancers won't see us as potential competition instead of a potential ally? Can we trust them not to reveal out location? Even if we are all working together to conquer the world, I doubt it will be so simple as us peacefully splitting it between ourselves. Revealing ourselves before securing our own safety from the others seems foolish.
Three:
Is there a way to communicate with the others? If there were, perhaps we could speak with them ourselves, and some of our worries could be put to rest. At the very least, we would prefer to know their stance on you - how do we know they've even agreed to the this alliance? Or won't stab it in the back at the first opportunity for personal gain?
Finally:
We will work with you and share your knowledge, for so long as it will benefit us mutually. We do not have enough information to make a lasting commitment - though we appreciate your cause, caution is warranted in such situations. If pressed, mention that we DO have some information that could be of use in exchange of a commitment, perhaps in the place of a commitment - the location of an artifact. (Don't actually reveal it, just see how he responds for now)
We need to strongly consider what turning him down outright will mean - not only a lack of knowledge as a result, but perhaps more importantly a lack of influence. The other necromancers, regardless of their behaviour, may end up being potential allies later if their own goals don't line up with the mystery man here.