She didn't actually -say- she was another necromancer. I think she's some sort of god? She DID change into a he-elf when she found we were she-elfy.
"Join me" = "Worship me", ya?
She did say that she was a necromancer, but she didn't say she was a necromancer like us. It's possible that she's a necromantic god, but since there's been no mention of gods so far, I very much doubt it- religions in this world seem to focus on concepts such as Water and Order, not actual deities, although those concepts may have personifications for all we know.
As for the transformation, isn't strange that she made such a blatant demonstration of her powers, at least in this supposed dream realm? (She never actually said we were in there, just implied it strongly.) That's not exactly conducive to deception. There are two most likely reasons for this: Either it's such an ingrained habit that she simply didn't think of it (in which case, she must indeed be skilled at 'dreamscaping', and, if she's telling the truth about it being as difficult as it is, she's also probably a very skilled mage, as well) or she
wants us to be aware of her abilities. Perhaps this is a forewarning, so that we don't try anything funny- necromancers of the past seem to lend credence to this theory. On the other hand, it could be a literal demonstration- an attempt to both quail us and get us curious about her offer.
Either way, caution is the best course to take here- but don't get so paranoid that we disregard her without hearing what she has to say, or worse, make an enemy of her. We don't want to take a gift horse without looking in its mouth, not in our situation, but we also don't want to lose what may be our only opportunity to learn necromancy from someone who seems to know what they're doing, instead of through vague secondhand accounts and dangerous experimentation.
EDIT: As for a title, I, personally, think that's a little egoistic at this point, but whatevs.