For the key 4 in death valley, you need to do the achievement " insatiable thirst "
to do this achievment, you need, at the very beginning, to bomb a tomb
you'll also find the " terror of the werepig " there.
then, with the key 4, you'll find the " the dark wizard of ooze "
and the ruby amulet from the achievement " witchcraft "
and then yes, you can go talk to the necromencer, he will give you the key to his room and you'll get the new spell.