Didn't even notice that the thread was from 2018, and the conversation has gone on for a time now. In any case, the thread is still topical enough to be discussed.
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I doubt the use of a Shaman's ability to restore the (newly) dead would be seen explicitly as Necromancy, in the same way a Spiritwalker's abilities to commune with the dead probably also aren't explicitly taboo, though Spiritwalkers do seem to be somewhat feared or held in awe in Tauren society. That being said, in terms of lore there appears to some element of consent when it comes to being resurrected (as opposed to raised into undeath), and either Cairne would not or did not consent, or else it simply wasn't an option. Given that Cairne was poisoned by Magatha, it may well be that whatever she used to weaken him also prevented him from being returned to life.